chore(hooks): add prepare-commit-msg normalizer

[tannn]

Summary

• Adds git-hooks/prepare-commit-msg hook that automatically normalizes commit subject lines before the commit-msg validator runs
• Lowercases the description portion after the Conventional Commits prefix (e.g. feat(ui): Add X → feat(ui): add X)
• Strips a trailing period from the subject line (e.g. fix: handle nil pointer. → fix: handle nil pointer)

Behavior

The hook is a no-op in all safe cases:

• Merge commits (COMMIT_SOURCE=merge)
• Squash commits (COMMIT_SOURCE=squash)
• Fixup commits (subject starts with fixup! or squash!)
• Template-only messages (subject is empty or all comments)
• Messages that don't match a Conventional Commits prefix (won't guess at normalization)
• Messages that already conform (no changes needed)

No changes to scripts/setup-hooks.sh are needed — it already symlinks every file in git-hooks/ automatically.

Test plan

• Commit with feat(ui): Add Dark Mode Toggle. — should normalize to feat(ui): add dark mode toggle
• Commit with fix: handle nil pointer — should pass through unchanged
• Merge commit — hook should be skipped entirely
• Commit with no CC prefix — hook should be skipped entirely
• Run bash scripts/setup-hooks.sh and verify prepare-commit-msg symlink is created

🤖 Generated with Claude Code

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[tannn]

Iteration 2 — fixes from review:

• Critical fix: Replaced awk sub(old, new) with direct print new; done=1; next. The previous sub() call treated the subject line as an ERE regex, so parentheses in scopes like feat(ui): were interpreted as capturing groups, causing silent no-match and leaving the subject un-normalized.
• Minor fix: Updated comment from "Lowercases the description portion" → "Lowercases the first character of the description".
• Minor fix: Changed || true to || SUBJECT="" so grep I/O errors still propagate while exit-1 (no match) is handled gracefully.