🐌 feat: skip auto-commit while a manual commit is in progress

[sdougbrown]

In read-write mode the daemon stages and auto-commits on its interval. When a manual git commit was mid-composition in an editor, the daemon could commit the same staged content with a generated message and push, tending the repo out from under the user. Git holds no lock during the editor phase, so this change detects an in-progress commit via a fresh .git/COMMIT_EDITMSG and backs off.

• internal/git: add GitDir, ActiveLocks, CommitEditRecent, ClearCommitEditMsg.
• internal/sync: skip auto-commit in read-write mode while any .git/*.lock is present; when in_progress_window is set, also skip while COMMIT_EDITMSG is fresh; clear COMMIT_EDITMSG after git-tend's own auto-commit so it does not back off against itself.
• internal/config: add commit.in_progress_window (TOML string duration, validated).
• README.md: document the new option and its trade-off.

Behavior is opt-in and off by default; existing read-write behavior is unchanged unless in_progress_window is set.

Validation:

• go build ./... — ok
• go test ./internal/git/ ./internal/sync/ ./internal/config/ — pass
• go test ./... — pass; one pre-existing failure in cmd/git-tend (TestRunStatusEmptyRepos) also fails on main, unrelated to this change

Known limitation: because git leaves COMMIT_EDITMSG around after a finished commit, enabling this also leaves a quiet window after a manual commit before auto-commit resumes. The window length is user-controlled.

[umpire-bot]

This PR is marked... SAFE! 🙌

Must fix

• [confidence: 95] internal/git/git_test.go - ClearCommitEditMsg is not tested, leaving its behavior for both existing and non-existing COMMIT_EDITMSG files unverified.
• [confidence: 90] internal/sync/sync_test.go - syncReadWrite does not test the case where git.ActiveLocks returns true, failing to verify the new integration branch that skips synchronization when a git lock is detected.

Should fix

• [confidence: 75] internal/git/git_test.go - GitDir error paths (e.g., when the underlying git command fails) are not explicitly tested.

Must fix

• [confidence: 95] internal/git/git_test.go: ClearCommitEditMsg is not tested. This function is used in the core sync loop to clean up the COMMIT_EDITMSG sentinel after an auto-commit; its behavior in both successful removal and non-existent file scenarios is unverified.
• [confidence: 90] internal/sync/sync_test.go: The branch in syncReadWrite that handles git.ActiveLocks returning true is not covered. A test should be added to verify that Sync returns skipped when a .lock file is detected in the repository's .git directory.

Should fix

• [confidence: 75] internal/git/git_test.go: GitDir is not tested in isolation. As an exported function, its error paths (e.g. when the underlying git command fails) should be explicitly exercised.