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>

tests/unit/test_turn_runtime.py

@@ -0,0 +1,36 @@
+"""R04-T04 — unit tests for the shared turn-runtime helpers.
+
+``combine_instructions`` is the small rule the UI applied inline: a turn's
+standing instructions are several independent blocks (project context, an Admin
+agent's persona, a skill's rules, an unattended-run reminder) that must be joined
+with one blank line, skipping whatever is absent. Two call sites need it (T04's
+widget and T05's task runner), which is exactly when a rule stops being an inline
+expression.
+"""
+
+from __future__ import annotations
+
+from cowork_local.application.conversations.turn_runtime import combine_instructions
+
+
+def test_two_blocks_are_joined_by_a_blank_line() -> None:
+    assert combine_instructions("PROJECT", "AGENT") == "PROJECT\n\nAGENT"
+
+
+def test_an_absent_block_leaves_no_blank_line_behind() -> None:
+    assert combine_instructions("", "AGENT") == "AGENT"
+    assert combine_instructions("PROJECT", "") == "PROJECT"
+    assert combine_instructions("PROJECT", None) == "PROJECT"
+
+
+def test_whitespace_only_blocks_do_not_count_as_instructions() -> None:
+    assert combine_instructions("   \n ", "AGENT") == "AGENT"
+
+
+def test_nothing_to_say_produces_an_empty_string() -> None:
+    assert combine_instructions() == ""
+    assert combine_instructions("", None, "  ") == ""
+
+
+def test_more_than_two_blocks_keep_their_order() -> None:
+    assert combine_instructions("A", "B", "C") == "A\n\nB\n\nC"