feat: add commit message normalizer

[marcus]

Summary

• add reusable commit message normalization helpers and a td commit-message / td commit-msg command
• install a worktree-safe commit-msg hook alongside the existing pre-commit hook
• document the normalized commit workflow and cover hook behavior, suffix handling, and skip rules with tests

Testing

• go test ./internal/git ./cmd
• manual smoke pass in a temp repo/worktree for focused issue normalization, explicit docs no-issue commits, duplicate suffix cleanup, malformed suffix rejection, and hook rewriting

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