feat(R06): workspace session snapshot, atomic persistence, history-dir race fix

EPIC R06 (Team Hoa) - workspace/filesystem isolation, no cross-project
mutable state.

R06-T01 domain/workspaces/workspace_session.py
  WorkspaceSession - project_id/workspace_root/sandbox_dir/allowed_paths
  frozen snapshot + is_allowed(path), same "capture once at submit time"
  shape as R04's ConversationExecutionRequest.

R06-T02 infrastructure/persistence/json/{atomic_write,workspace_repository_impl,conversation_repository_impl}.py
  Real bug fixed: core/projects.py::save_project and core/history.py's
  save_conversation/rename_conversation/set_pinned did a plain
  path.write_text(json.dumps(...)) - two syscalls, no atomicity. A crash
  between them leaves a half-written file that load_project/load_conversation
  then silently treat as "missing". All four now write through
  atomic_write.write_json (temp file + os.replace). WorkspaceRepository/
  ConversationRepository are thin object-shaped facades over the same
  (now-atomic) functions, for future application-layer callers.
  NOTE: atomic_write.py is deliberately NOT named atomic_json_file.py -
  R02-T01 (Team Nam) claims that filename for the same purpose app-wide;
  see the checklist for the consolidation TODO.

R06-T03 infrastructure/filesystem/execution_workspace.py
  ExecutionWorkspace names the output_dir/scratch_dir split that already
  exists (core/chat_agent.py's flat workspace_root/.scratch) - does not
  move anything.

R06-T04 ui/chat_panel.py
  The actual race: ChatPanel._persist_session (saves a BACKGROUND turn's
  conversation) resolved its save directory via a live
  self.ctx.config.history_dir() read at save time. ui/workspace_tab.py::
  _load_current mutates that same config field on every project switch, so
  a turn still running when the user switched projects got saved into the
  NEW project's history folder. Fixed by adding "home_history_dir" to the
  per-turn ctx dict (same "home_*" snapshot convention already used for
  session id/messages/title), captured at submit time. Verified with a real
  offscreen-Qt test, not just a unit double:
  tests/integration/test_history_dir_race.py.

R06-T05 application/workspaces/file_workspace_service.py
  FileWorkspaceService - the File Explorer / AI Editor entry point for the
  same safe read/write/edit operations the agent tool loop has, by calling
  core/tools.py::execute_tool directly (same dispatch, same ToolContext
  containment, same audit log) rather than reimplementing any of it.

New tests: tests/unit/test_workspace_session.py,
test_atomic_write_and_repositories.py, test_execution_workspace.py,
test_file_workspace_service.py, tests/integration/test_history_dir_race.py
(29 new tests, incl. 2 real offscreen-Qt integration tests).

Suite: 283 passed, 4 pre-existing failures unrelated to R05/R06 (see
checklist). check_imports: PASS. All new files < 400 LOC.

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

infrastructure/persistence/json/conversation_repository_impl.py

@@ -0,0 +1,54 @@
+"""ConversationRepository - an object-shaped, atomic-write-backed facade over
+``core/history.py`` (R06-T02). Same rationale as
+``workspace_repository_impl.py``: the module-level functions in
+``core/history.py`` are still what production code calls (they now write
+atomically themselves), this class is the seam for application-layer code
+that wants an object instead of a directory-parameterised function.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any, Dict, List, Optional
+
+from cowork_local.config import HISTORY_DIR
+from cowork_local.core.history import (
+    delete_conversation,
+    list_conversations,
+    load_conversation,
+    new_session_id,
+    rename_conversation,
+    save_conversation,
+    set_pinned,
+)
+
+
+class ConversationRepository:
+    """CRUD + search over conversation JSON files, scoped to one
+    ``directory`` (defaults to the app's real ``HISTORY_DIR``)."""
+
+    def __init__(self, directory: Optional[Path] = None) -> None:
+        self._directory = Path(directory) if directory is not None else HISTORY_DIR
+
+    def new_session_id(self) -> str:
+        return new_session_id()
+
+    def save(self, kind: str, session_id: str, messages: List[Dict[str, Any]], **kwargs) -> Path:
+        return save_conversation(self._directory, kind, session_id, messages, **kwargs)
+
+    def load(self, path: Path) -> Dict[str, Any]:
+        return load_conversation(path)
+
+    def list(self, query: str = "") -> List[Dict[str, Any]]:
+        return list_conversations(self._directory, query)
+
+    def delete(self, path: Path) -> None:
+        delete_conversation(path)
+
+    def rename(self, path: Path, new_title: str) -> None:
+        rename_conversation(path, new_title)
+
+    def set_pinned(self, path: Path, pinned: bool) -> None:
+        set_pinned(path, pinned)
+
+
+__all__ = ["ConversationRepository"]