feat(R04): add the immutable turn snapshot and typed agent event stream

R04-T01 — `domain/agents/conversation_execution_request.py`: a frozen
snapshot of everything one chat turn needs. Turn inputs previously lived in a
closure plus a 15-key ctx dict inside `ui/chat_panel.py::_start_turn`, and the
worker thread kept reading the widget back while it ran, so every later click
was visible to work already in flight. The request also owns the prompt
composition rules (instruction prefix separator, session notes, model-switch
review note) that were inline in that closure.

R04-T02 — `domain/agents/agent_event.py`: 13 frozen event types replacing the
untyped `{"type": ...}` dicts, whose only specification was the 130-line
if/elif chain in `_on_event`. Each event serialises back to the exact legacy
dict, so the presentation layer is untouched; `agent_event_codec.py` parses the
other way and is a temporary shim, isolated so R08 can delete it in one move.
`assistant_done` is deliberately NOT the end of a turn (it fires once per
provider call), so it maps to AssistantMessageCompletedEvent while the new
TurnCompletedEvent reports the turn itself.

R04-T03 (part) — `domain/agents/agent_result.py`: one named outcome for a
finished turn, replacing the message list / 3-tuple / reconstructed-from-side-
effects trio the three callers each read differently.

Verification: 66 tests. Beyond the unit tests,
`tests/integration/test_agent_event_bridge.py` runs the REAL `run_cowork` loop
offline and asserts every dict it emits is recognised and round-trips
byte-for-byte — a guard against an event type nobody modelled or a key whose
meaning silently drifted.

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

tests/integration/test_agent_event_bridge.py

@@ -0,0 +1,113 @@
+"""R04-T02 — the typed event vocabulary vs. what the real runtime emits.
+
+The unit tests pin each event against the shape I *read* out of
+``core/chat_agent.py``. This one removes the reading: it runs the actual
+``run_cowork`` loop offline (FakeProvider, real tool execution, real cleanup)
+and asserts every dict it emits is recognised by :func:`from_legacy_dict` and
+survives a round trip byte-for-byte.
+
+That makes it a guard against the two failure modes a hand-written vocabulary
+has: an event type nobody modelled, and a key that silently changes meaning.
+Either one would surface here as a failure instead of as a blank chat bubble
+after R04-T03 starts routing events through the typed layer.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any, Dict, List
+
+import pytest
+from cowork_local.core import chat_agent
+from cowork_local.domain.agents.agent_event_codec import from_legacy_dict
+from cowork_local.tests.fakes.fake_provider import FakeProvider
+
+
+def _run_turn_and_collect(tmp_path: Path, provider: FakeProvider) -> List[Dict[str, Any]]:
+    """Run one real ``run_cowork`` turn offline and return every emitted dict."""
+    output_dir = tmp_path / "output"
+    output_dir.mkdir(parents=True, exist_ok=True)
+    emitted: List[Dict[str, Any]] = []
+
+    chat_agent.run_cowork(
+        provider=provider,
+        messages=[{"role": "user", "content": "make me a report"}],
+        output_dir=output_dir,
+        emit=emitted.append,
+        # security_config=None disables the AI guardrail layers, which is the
+        # documented behaviour for headless callers and keeps this test offline.
+        security_config=None,
+        title="Report",
+    )
+    return emitted
+
+
+def _reporting_turn(tmp_path: Path) -> List[Dict[str, Any]]:
+    """A turn that streams text, calls save_file, then answers — the common path."""
+    provider = FakeProvider()
+    provider.queue_response(
+        content="Writing it now.",
+        chunks=["Writing ", "it now."],
+        tool_calls=[{"id": "call_1", "name": "save_file",
+                     "arguments": {"filename": "report.md", "content": "# Report\n"}}],
+    )
+    provider.queue_response(content="Saved to report.md.", chunks=["Saved to report.md."])
+    return _run_turn_and_collect(tmp_path, provider)
+
+
+def test_the_runtime_emits_only_event_types_the_domain_layer_models(tmp_path: Path) -> None:
+    emitted = _reporting_turn(tmp_path)
+
+    unmodelled = sorted({e["type"] for e in emitted if from_legacy_dict(e) is None})
+
+    assert unmodelled == [], f"run_cowork emits event types R04-T02 does not model: {unmodelled}"
+
+
+def test_every_emitted_event_round_trips_without_losing_a_key(tmp_path: Path) -> None:
+    emitted = _reporting_turn(tmp_path)
+    assert emitted, "the turn produced no events at all — the fixture is wrong"
+
+    for raw in emitted:
+        event = from_legacy_dict(raw)
+        assert event is not None, raw
+        assert event.to_legacy_dict() == raw, f"round trip changed the {raw['type']} event"
+
+
+def test_a_tool_using_turn_really_exercises_the_tool_events(tmp_path: Path) -> None:
+    # Guards the test above from passing trivially: if the fixture ever stopped
+    # calling a tool, the round-trip check would only cover text events.
+    types = {e["type"] for e in _reporting_turn(tmp_path)}
+
+    assert {"text", "assistant_done", "tool_proposed", "tool_result"} <= types
+
+
+def test_reasoning_events_from_a_thinking_model_round_trip(tmp_path: Path) -> None:
+    # A separate fixture because only reasoning models emit these, and the
+    # common-path turn above would otherwise never cover the event.
+    provider = FakeProvider()
+    provider.queue_response(content="42", chunks=["42"], reasoning="Let me think...")
+
+    emitted = _run_turn_and_collect(tmp_path, provider)
+
+    reasoning_events = [e for e in emitted if e["type"] == "reasoning"]
+    assert reasoning_events, "a reasoning model produced no reasoning event"
+    for raw in reasoning_events:
+        assert from_legacy_dict(raw).to_legacy_dict() == raw
+
+
+def test_plan_events_from_the_real_update_plan_tool_round_trip(tmp_path: Path) -> None:
+    provider = FakeProvider()
+    provider.queue_response(
+        content="Planning.",
+        tool_calls=[{"id": "call_1", "name": "update_plan",
+                     "arguments": {"steps": [{"title": "Draft", "status": "running"},
+                                             {"title": "Review", "status": "pending"}]}}],
+    )
+    provider.queue_response(content="Done.")
+
+    emitted = _run_turn_and_collect(tmp_path, provider)
+
+    plan_events = [e for e in emitted if e["type"] == "plan_set"]
+    assert plan_events, "update_plan did not produce a plan_set event"
+    for raw in plan_events:
+        assert from_legacy_dict(raw).to_legacy_dict() == raw