Fix commit-message hook edge cases

[marcus]

Summary

• add a td-native commit-message normalizer command and reusable internal parser/rewriter
• preserve merge and autosquash subjects while keeping commit bodies and trailers untouched
• make make install-hooks safe from linked worktrees and align contributor docs on the canonical workflow

Testing

• go test ./internal/git
• go test ./cmd -run 'TestCommitMessage|TestInstallHooks'
• go test ./...
  • note: this still hits the pre-existing failure in test/e2e/TestChaosSync with seed 1775903130835843000, reproduced on clean main as well
• manual td commit-message --file checks for normalized, fixup, and merge subjects

Supersedes the earlier v14 attempt / PR #128 by addressing the linked-worktree, autosquash, and merge-message regressions from review.

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