fix(workflow): invalidate subgraph cache on YAML file edit (#622)

[eldar702]

Problem

Fixes #622. Editing a workflow YAML used by a subgraph node and re-running the subgraph executes the old version; only restarting ChatDev picks up the change.

workflow/subgraph_loader.py caches parsed subgraphs in a module-level dict _SUBGRAPH_CACHE keyed only on the resolved file path, with no staleness check:

if resolved_path not in _SUBGRAPH_CACHE:
    _SUBGRAPH_CACHE[resolved_path] = _load_graph_dict(resolved_path)
payload = _SUBGRAPH_CACHE[resolved_path]

Once a subgraph is loaded, the on-disk YAML is never re-read for the process lifetime — exactly the reporter's "stale until restart" symptom.

Fix

workflow/subgraph_loader.py — store the file's st_mtime alongside the cached payload and reload when the on-disk mtime changes:

mtime = resolved_path.stat().st_mtime
cached = _SUBGRAPH_CACHE.get(resolved_path)
if cached is None or cached[0] != mtime:
    _SUBGRAPH_CACHE[resolved_path] = (mtime, _load_graph_dict(resolved_path))
payload = _SUBGRAPH_CACHE[resolved_path][1]

The cache is preserved for unedited files (no extra parse on the common path) — the only new behavior is invalidation when the file actually changes. No new dependencies. 12-line change in 1 source file.

Test (RED-first)

Adds tests/test_subgraph_loader_cache.py:

1. test_reload_after_edit_returns_new_content — write a subgraph YAML, load it (asserts v1), edit the same file (bumping mtime via os.utime to defeat coarse-clock filesystems), load again, assert it now reflects v2. This test is RED on main (returns the cached v1) and GREEN with this fix.
2. test_unchanged_file_is_served_from_cache — proves an unedited file is not re-parsed, so the perf-oriented cache is retained.

Verified RED→GREEN locally:

# before the fix
tests/test_subgraph_loader_cache.py::test_reload_after_edit_returns_new_content FAILED
  AssertionError: subgraph_loader served a stale cached copy after the file was edited
  assert 'v1' == 'v2'

# after the fix
tests/test_subgraph_loader_cache.py::test_reload_after_edit_returns_new_content PASSED
tests/test_subgraph_loader_cache.py::test_unchanged_file_is_served_from_cache PASSED

Verification

• uv run pytest tests/test_subgraph_loader_cache.py → 2 passed.
• uv run pytest (excluding the pre-existing-on-main hanging tests/test_websocket_send_message_sync.py) → 56 passed. That websocket test hangs identically on a pristine main checkout and is untouched by this PR.

🤖 AI-assistance: Claude (Opus 4.8), reviewed