fix(auto-capture): locate a commit at its code, not at .projectmem/ files

[RTE404]

Fixes #20.

Auto-captured commits used files[0] as their location. git lists changed paths sorted, .projectmem/ sorts ahead of most paths, and summary.md is regenerated on every event, so most commits were located at .projectmem/summary.md. The staleness check then counted every later commit to summary.md against those events and flagged nearly all of them as possibly stale.

Change: commands/auto_capture.py now picks the first changed path outside .projectmem/ as the location. A commit that touches only memory files gets no location, so it is never flagged against summary.md either. files is unchanged: the event still lists every path the commit touched.

Tests: new tests/test_auto_capture.py makes a real git repo and runs _capture_commit against it:

• a commit touching .projectmem/summary.md and src/login.py is located at src/login.py, and its files still include summary.md;
• a commit touching only .projectmem/summary.md has no location.

Both tests fail on main and pass with this change.

Verification (Windows 11, Python 3.12):

• Full suite: 277 passed, 4 failed. The same 4 fail on main without this change (test_global_mcp.py ×3, test_init_ux.py::test_print_mcp_config_single_project_still_pins_the_root), so they're pre-existing on Windows and unrelated. The two MCP stdio tests in test_mcp_server.py time out intermittently on this machine on both main and this branch.
• ruff check: no new findings. auto_capture.py reports the same 9 as on main; tests/test_auto_capture.py is clean.
• Merges cleanly with current main.

Notes:

• Events captured before this fix keep their old location, since the log is append-only, so existing stale flags on .projectmem/ files will persist.
• Separate, not changed here: git diff-tree without --root lists no files for a repo's first commit, so that commit is captured with no files. Happy to send a follow-up if that's wanted.

[riponcm]

Thanks Rohan, good catch, and confirmed: the new tests fail on main and pass with your change. I'll merge this in the next update, planned for this Sunday. And yes, a follow-up for the first-commit --root case would be welcome.