feat(R04): immutable turn snapshot, typed agent events, conversation service

EPIC R04 (Team Duy) - the turn lifecycle leaves the widget.

R04-T01 domain/agents/conversation_execution_request.py
  Frozen snapshot of one turn, captured on the UI thread at submit time. The
  job closure used to read widget/workspace state from inside the worker
  thread, so a turn could run on a mix of submit-time and later state
  depending on thread timing.
R04-T02 domain/agents/agent_event.py
  13 frozen event types replacing untyped emit() dicts, with a two-way bridge
  so existing widgets keep consuming the legacy shape until EPIC R08. Adds
  TurnCompletedEvent - the end-of-turn signal the engine never had, which is
  why a cancelled turn and a failed turn look identical to the UI today.
R04-T03 application/conversations/conversation_application_service.py
  Runs a turn from a request and reports typed events. Never raises across the
  worker boundary; TurnResult.raise_if_failed() preserves the existing
  exception-based failure path. begin_turn()/execute_turn() expose the live
  message list for callers that autosave history mid-run.
R04-T04 ui/cowork_tab.py::build_job -> snapshot + service.
R04-T05 core/task_executors.py::_run_agent -> same service (was a second,
  slightly different assembly of the same call).

Caught while wiring the bridge: the first event vocabulary had no "notice"
event, so Agent Security warnings and auto-compaction notices would have been
silently swallowed. Added NoticeEvent plus a test that scans the engine sources
for emit() tags and fails when one has no typed counterpart.

New: tests/integration/ - real offscreen CoworkTab running a scripted turn end
to end (7 tests), including a characterisation of the extra provider call Agent
Security spends reviewing each request.

Suite: 225 passed, 2.74s. check_imports: PASS. All new files < 400 LOC.
2 pre-existing failures remain in test_config_security.py (EPIC R02/Team Nam).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

domain/agents/__init__.py

@@ -1,2 +1,48 @@
-"""Domain entities for one agent turn: the request snapshot and the event
+"""Domain entities for one agent turn: the request snapshot and the typed event
 stream it produces (EPIC R04)."""
+
+from .agent_event import (
+    AgentEvent,
+    AssistantDoneEvent,
+    ErrorEvent,
+    HistoryReadyEvent,
+    NoticeEvent,
+    OutputsAddedEvent,
+    OutputsRemovedEvent,
+    PlanUpdatedEvent,
+    ReasoningChunkEvent,
+    TextChunkEvent,
+    ToolCallFinishedEvent,
+    ToolCallStartedEvent,
+    ToolOutputEvent,
+    TurnCompletedEvent,
+    collect_text,
+    event_from_dict,
+    tool_calls,
+)
+from .conversation_execution_request import (
+    ConversationExecutionRequest,
+    new_turn_id,
+)
+
+__all__ = [
+    "ConversationExecutionRequest",
+    "new_turn_id",
+    "AgentEvent",
+    "TextChunkEvent",
+    "ReasoningChunkEvent",
+    "AssistantDoneEvent",
+    "PlanUpdatedEvent",
+    "ToolCallStartedEvent",
+    "ToolOutputEvent",
+    "ToolCallFinishedEvent",
+    "OutputsAddedEvent",
+    "OutputsRemovedEvent",
+    "NoticeEvent",
+    "HistoryReadyEvent",
+    "TurnCompletedEvent",
+    "ErrorEvent",
+    "event_from_dict",
+    "collect_text",
+    "tool_calls",
+]