Adds a new commit search operator alias (is:tip / type:tip) and associated filtering so both the general commit search and the Commit Graph can be narrowed to commits that are branch or tag tips.

Summary

Add ability to filter and search commits (including the Commit Graph) to only branch & tag tip commits using is:tip (alias type:tip).

Impact

Improves precision for workflows focused on release/tag coordination, branch divergence analysis, and scanning strategic heads without traversing full histories. Users can quickly isolate meaningful entry-point commits rather than manually identifying heads among large histories, reducing cognitive load and time spent filtering visually.

Validation

1. Open commit search UI; enter is:tip and verify only branch/tag tip commits are returned.
2. Replace with type:tip and confirm identical results (alias works).
3. Remove the operator and ensure broader set of commits appear (regression check).
4. Combine with other operators (e.g. author:, after:, ref:) to ensure intersection works without errors.
5. Use is:stash to confirm stash filtering still functions and is mutually exclusive with is:tip.
6. Open Commit Graph with is:tip filter applied; ensure graph reflects only tip commits (no mid-branch commits).
7. Confirm performance is not degraded (no noticeable slowdown vs previous search).

Risk

Low. Changes are additive and guarded by explicit filter usage. Existing searches remain unaffected. Primary risk is misclassification if tip detection (%D) parsing fails; mitigated by limiting parsing scope and leaving default path untouched. No data mutation or destructive operations introduced.