fix(xtask): perf attestation fails in git worktrees

[rocketman-code]

Current Behavior

The pre-commit, pre-push hooks and perf-validate command hardcode .git/perf-attestation.json and .git/hooks/ paths. In a git worktree, .git is a file (pointer to the real git dir), not a directory. This causes:

1. cargo xtask perf-validate fails to write attestation: Not a directory (os error 20)
2. Pre-push hook can't find the attestation and blocks the push
3. cargo xtask install-hooks would fail in a worktree

Expected Behavior

Hooks and attestation should work correctly in both normal repos and git worktrees.

Technical Details

Relevant Code

xtask/src/hooks.rs:54,85,118 and xtask/src/perf_validate.rs:246

All use root.join(".git/...") which assumes .git is a directory.

Root Cause

In a git worktree, .git is a file containing gitdir: /path/to/real/.git/worktrees/<name>. The fix is to use git rev-parse --git-dir which returns the correct directory in both contexts.