feat(R04): run every Cowork turn through ConversationApplicationService

R04-T03 — the turn lifecycle, extracted from `core/chat_agent.py::run_cowork`
into `application/conversations/`. The 260-line body mixed the lifecycle (step
budget, cancel checks, guard -> preview -> gate -> execute ordering, sandbox
tidy-up) with the machinery doing each step, and reaching any of it meant
standing up a Qt widget and a worker thread. It is now a plain object driven
through two Protocols and six callables (`turn_runtime.py`), with the concrete
`core/*` wiring confined to `core_runtime_adapter.py` — the same shape R03 used
for routing. Faithful port, not an improvement pass: where the original had a
quirk (the step-ceiling note only merges into the answer when the last message
is the assistant's) the quirk is preserved and commented.

R04-T04 — `ui/cowork_tab.py::build_job` no longer calls run_cowork. It captures
the widget's state at submit time, builds the request via the new
`cowork_turn_request.py` and executes it. `execute(..., messages=...)` hands the
widget's own list over because `_reattach_running_turn` replays from it WHILE
the worker appends and `_finalize_turn` slices it afterwards — a private list
would break both silently.

R04-T05 — `core/task_executors.py`'s cowork branch shares the same engine. All
five unattended-run behaviours stay put (plan reminder, history_ready, History
autosave per assistant message, timeout notice, plan_incomplete_reason), and
`_unattended_prompt` now expresses the load-bearing prefix order in one
readable call instead of three successive rebindings.

Verification: 74 new tests (364 passed, 1 skipped overall; check_imports PASS).
The two that matter most:
- `test_conversation_service_parity.py` runs the same scripted turn through
  run_cowork AND the service and compares the event stream, the resulting
  conversation and the advertised tool list across 7 scenarios;
- `test_task_executor_turn.py` was written BEFORE the migration and passed 8/8
  against the old code, then unchanged against the new.

Known: `ui/cowork_tab.py` (416 -> 455) and `core/task_executors.py` (476 -> 524)
stay above the 400-LOC limit. Both were already over it before this change;
bringing them under needs the R08 / R07 decompositions.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/task_executors.py

@@ -2,7 +2,9 @@
 
 ``execute_task`` dispatches by ``task_type`` to the app's existing engines:
 
-- ``cowork``     → ``chat_agent.run_cowork``  (documents/answers, real files)
+- ``cowork``     → ``ConversationApplicationService`` (documents/answers, real
+                   files) — the same turn engine the interactive Cowork chat
+                   runs on since R04-T05
 - ``co4e_code``  → ``code_agent.run_code``    (code agent with file/command tools)
 - ``script``     → local subprocess with a timeout
 - ``flow``       → the task's own simple step list, run sequentially, each
@@ -162,6 +164,30 @@ _TIMEOUT_NOTICE_TMPL = (
 )
 
 
+_UNATTENDED_PREFIX = (
+    "This runs unattended (Schedule Task) — no one is watching live. Use "
+    "update_plan to track your steps and keep it accurate: mark a step "
+    "'error' (not silently skip it) if it genuinely can't be completed."
+)
+
+
+def _unattended_prompt(prompt: str, *, skill_text: str = "",
+                       agent_instructions: str = "") -> str:
+    """Assemble the user message an unattended run sends.
+
+    The order is load-bearing and used to be encoded as three successive
+    rebindings of ``prompt``, each prepending its own block: the plan reminder
+    must lead (it is the instruction that keeps a run without a human watching
+    honest), then the chosen skill's rules, then the Admin agent's persona, and
+    the task's own words last. Routing it through ``combine_instructions`` keeps
+    that order in one readable expression and drops the absent blocks instead of
+    leaving blank lines behind.
+    """
+    from ..application.conversations.turn_runtime import combine_instructions
+
+    return combine_instructions(_UNATTENDED_PREFIX, skill_text, agent_instructions, prompt)
+
+
 def _cancel_with_timeout(cancel: CancelFn, timeout_sec: Optional[int]) -> Tuple[CancelFn, Callable[[], bool]]:
     """Wrap ``cancel`` so it also fires once ``timeout_sec`` of wall-clock time
     elapses. ``timed_out()`` tells the caller whether THAT is why it stopped
@@ -218,36 +244,29 @@ def _run_agent(ctx, task_type: str, prompt: str, out_dir: Path,
     # default, see state.build_provider_for). A legacy Admin-agent preset
     # (task.admin_agent_id), if still set on an older task, keeps working and
     # takes precedence — it pins the provider/model AND prepends instructions.
+    agent_instructions = ""
     if admin_agent is not None:
         from .admin_agents import build_agent_provider
 
         provider = build_agent_provider(ctx, admin_agent)
         agent_instructions = admin_agent.effective_prompt()
-        if agent_instructions:
-            prompt = f"{agent_instructions}\n\n{prompt}"
     elif provider_name or model:
         # An explicit per-task provider/model override.
         provider = ctx.build_provider_for(provider_name or None, model or None)
     else:
         # Neither overridden → the machine's own Settings default, exactly as before.
         provider = ctx.build_active_provider()
-    # A chosen skill's instructions are prepended so this unattended run follows
+    # A chosen skill's instructions are applied so this unattended run follows
     # them, mirroring how the interactive chat applies /skill.
+    skill_text = ""
     if skill_slug:
         from .skills import skill_prefix_for
 
         skill_text = skill_prefix_for(skill_slug)
-        if skill_text:
-            prompt = f"{skill_text}\n\n{prompt}"
-    # This is an UNATTENDED run (no human watching to catch a half-finished
-    # job) — push the agent to actually use the Plan checklist so completion
-    # can be verified afterward, instead of just trusting "no exception".
-    prompt = (
-        "This runs unattended (Schedule Task) — no one is watching live. Use "
-        "update_plan to track your steps and keep it accurate: mark a step "
-        "'error' (not silently skip it) if it genuinely can't be completed.\n\n"
-        f"{prompt}"
-    )
+    # Assemble reminder + skill + persona + the task's own words in one place
+    # (see _unattended_prompt for why that order matters).
+    prompt = _unattended_prompt(prompt, skill_text=skill_text,
+                                agent_instructions=agent_instructions)
     messages = [{"role": "user", "content": prompt}]
     session_id = new_session_id()
     project_id = project.project_id if project is not None else ""
@@ -273,10 +292,41 @@ def _run_agent(ctx, task_type: str, prompt: str, out_dir: Path,
     watched_cancel, timed_out = _cancel_with_timeout(cancel, timeout_sec)
     try:
         if task_type == "cowork":
-            from .chat_agent import run_cowork
-            run_cowork(provider, messages, out_dir, emit_and_autosave, watched_cancel,
-                      security_config=ctx.config, agent_role=agent_roles.TASK,
-                      project_context=project_context)
+            # R04-T05: the unattended run shares the interactive turn engine
+            # instead of calling run_cowork itself, so there is exactly one place
+            # where a turn's lifecycle is defined. Everything unattended-specific
+            # stays here (the plan reminder above, the History autosave in
+            # emit_and_autosave, the timeout notice below).
+            from ..application.conversations.core_runtime_adapter import (
+                build_cowork_conversation_service,
+                legacy_event_sink,
+            )
+            from ..domain.agents.conversation_execution_request import (
+                ConversationExecutionRequest,
+            )
+
+            # No extra_tools/extra_executor and no permission gate: a scheduled
+            # run gets no MCP connectors and nobody is there to approve a
+            # command, which is exactly what run_cowork was called with.
+            service = build_cowork_conversation_service(
+                provider, out_dir, emit_and_autosave, title=title,
+                project_context=project_context, security_config=ctx.config,
+                agent_role=agent_roles.TASK,
+            )
+            request = ConversationExecutionRequest(
+                # The artifact folder is named by the run id, which identifies
+                # this attempt in the audit log.
+                turn_id=out_dir.name or session_id, session_id=session_id,
+                surface="task", title=title, project_id=project_id,
+                prompt=prompt, output_dir=out_dir,
+                agent_role=agent_roles.TASK, unattended=True,
+                timeout_sec=timeout_sec,
+            )
+            # ``messages`` is handed over so the History autosave in
+            # emit_and_autosave (and the final save in the finally block below)
+            # keep reading the live conversation as it grows.
+            service.execute(request, legacy_event_sink(emit_and_autosave),
+                            cancel=watched_cancel, messages=messages)
         else:
             from .code_agent import run_code
             limits, block_network = agent_security.sandbox_settings(ctx.config)