fix(memory): preserve watched file mtimes

[BunnyTeddy]

Fixes #649

Summary

MemorySyncManager now passes the filesystem mtime already captured by its watcher to LongTermMemoryStore.index_file() for both watched memory files and knowledge-base documents. This matches the direct ingestion path and keeps persisted freshness metadata and retrieval recency signals tied to the source file instead of the sync time. No extra filesystem stat is performed. Public APIs, schema, persistence, and migrations are unchanged.

Tests

• Added an offline regression test covering both MEMORY.md and knowledge_base/guide.md with deterministic mtimes.
• Focused memory suite: 17 passed.
• Control UI build, frontend check (98 files / 2,004 tests), ruff, mypy, and uv build --wheel pass.
• Local full pytest: 8,559 passed, 32 skipped; 6 unrelated ONNX/packaging failures and 3 ONNX fixture errors because this checkout's model asset is invalid and its uv does not support the test helper's --clear flag.

Diff is limited to src/agentos/memory/sync_manager.py and tests/test_memory_sync_manager_architecture.py.

[BunnyTeddy]

Addressed the accepted scope notes from #649:

• reuses the mtime already captured in MemorySyncManager._mtimes (no extra stat() call);
• passes that mtime to index_file() for both watched memory files and knowledge-base documents;
• extends the offline regression test to cover both MEMORY.md and knowledge_base/guide.md.

I also rebased onto the latest main (47102d6). Focused memory tests pass (17 passed), along with targeted ruff and mypy.

[andreapn]

LGTM