Add support for git worktrees

[kpaxton]

• Fix resolution of git root directory to support worktree folder structures
• Use the resolved gitRoot path for commitmsg parsing and modification

[KalvinWillison]

Tested this against a repo where I run Claude Code / CI work inside git worktree add-created worktrees — it correctly reads the worktree's own HEAD instead of the main checkout's. Reproducible on master today: the hook prepends the main checkout's ticket ID to commits made from a worktree, which is exactly the bug.

FWIW there's also #747 attempting the same fix — IMO this one (#746) is cleaner (relies on git's own worktree-aware --git-dir rather than adding an isInsideWorktree heuristic). Related: #749.

@bk201- — any chance of a look? This bug bites anyone using Claude Code, VS Code multi-root, or other tooling that encourages worktrees.

[rutgersc]

@bk201- Now with more usage of agents I can see this biting more users than before, could you look into this fix?