fix: support linked Git worktrees

What changed

• resolve .git/* commit-message paths through git rev-parse --git-path
• resolve the effective hooks directory for main checkouts and linked worktrees
• install and remove hooks through that resolved directory
• make this repository's own commit-msg hook worktree-safe
• add real linked-worktree integration tests

Why

Git worktrees use a .git pointer file. Treating it as a directory caused gitlint --edit .git/COMMIT_EDITMSG to fail with ENOTDIR, blocking valid commits in downstream Stacks worktrees.

Closes #550.

Validation

• bun run build
• bun test: 47 passed
• bun run typecheck
• bun run lint: 0 errors, 7 existing documentation warnings
• this commit itself passed the regenerated worktree-safe commit-msg hook