Feature/delta team/epic r04 (#7)

## Summary

epic r04 - begin refactor

## Change Type

- [x] Cowork feature
- [ ] Bug fix
- [ ] Core AI contribution
- [ ] Test / hardening
- [ ] Performance
- [ ] Documentation

## Related Work

Cowork Task:

Core Repo: http://34.143.229.138/gitea-admin/fsg-ai-core-assets

Core AI Issue:

Core Task:

Related PR:

## Scope

What is intentionally included?

What is intentionally NOT included?

## Validation

- [ ] Unit tests
- [ ] Integration tests
- [ ] Manual verification
- [ ] Regression check

Commands / evidence:

## Security Impact

Permission / credential / network / customer data impact:

## Compatibility

- [ ] No breaking change
- [ ] Breaking change documented

## Reviewer Notes

Anything Cowork reviewers should pay attention to.

---------

Co-authored-by: Anh Tran Nguyen Minh <anhtnm1@fpt.com>
Co-authored-by: Huong Le Thi Thien <huongltt35@fpt.com>
Co-authored-by: Nam Pham Dinh Thanh <nampdt@fpt.com>
Co-authored-by: Vu Dam Tuan <vudt15@fpt.com>
Co-authored-by: Hiep Ha Van <hiephv3@fpt.com>
Co-authored-by: Lam Hoang Van <lamhv7@fpt.com>
Reviewed-on: #7
Co-authored-by: Duy Le Huu <duylh19@fpt.com>

tests/unit/test_task_repository.py

@@ -0,0 +1,70 @@
+"""EPIC R07-T01: TaskRepository + core/tasks.py::save_task atomic write.
+
+The motivating bug: ``core/tasks.py::save_task`` used to
+``path.write_text(json.dumps(...))`` — two syscalls, no atomicity, same class
+of bug already fixed for projects/conversations at R06-T02. A failure between
+the write and the replace must never leave a half-written task JSON file on
+disk; that is the one property the crash-injection test exists to pin.
+"""
+from __future__ import annotations
+
+import json
+
+import pytest
+
+from cowork_local.core.tasks import new_task
+from cowork_local.infrastructure.persistence.json import TaskRepository
+
+
+def test_task_repository_crud_round_trip(tmp_path):
+    repo = TaskRepository(tmp_path)
+    task = repo.create("My Task")
+    repo.save(task)
+
+    assert [t["task_id"] for t in repo.list()] == [task["task_id"]]
+    assert repo.get(task["task_id"])["title"] == "My Task"
+
+    task["title"] = "Renamed"
+    repo.save(task)
+    assert repo.get(task["task_id"])["title"] == "Renamed"
+
+    repo.delete(task["task_id"])
+    assert repo.get(task["task_id"]) is None
+    assert repo.list() == []
+
+
+def test_task_repository_duplicate_keeps_config_resets_identity(tmp_path):
+    repo = TaskRepository(tmp_path)
+    task = repo.create("Original", description="d")
+    repo.save(task)
+
+    dup = repo.duplicate(task)
+    repo.save(dup)
+
+    assert dup["task_id"] != task["task_id"]
+    assert dup["description"] == "d"
+    assert {t["task_id"] for t in repo.list()} == {task["task_id"], dup["task_id"]}
+
+
+def test_save_task_never_corrupts_existing_file_on_crash(tmp_path, monkeypatch):
+    """Same guarantee as ``test_atomic_write_and_repositories.py``'s crash
+    test, exercised through ``core/tasks.py::save_task`` directly (not just
+    through the repository) since that is the function every existing
+    scheduler/executor call site still uses."""
+    from cowork_local.core.tasks import save_task, task_path
+
+    task = new_task("Stable")
+    save_task(task, tmp_path)
+
+    import cowork_local.infrastructure.persistence.json.atomic_write as mod
+
+    def boom(*_a, **_k):
+        raise OSError("simulated crash between write and replace")
+
+    monkeypatch.setattr(mod.os, "replace", boom)
+    task["title"] = "Corrupted?"
+    with pytest.raises(OSError):
+        save_task(task, tmp_path)
+
+    on_disk = json.loads(task_path(task["task_id"], tmp_path).read_text(encoding="utf-8"))
+    assert on_disk["title"] == "Stable"