chore: add commit message normalization

[marcus]

Detected convention

Recent history already mixes Conventional Commit subjects such as fix:, feat(tasks):, docs:, and refactor: with older free-form release commits. GoReleaser changelog filters also key off Conventional Commit prefixes for docs:, test:, ci:, and chore:, so this standardizes future commits on that format.

Implementation

• Added a root .gitmessage template documenting type(scope): description, allowed types, 72-character subjects, body/trailer spacing, and examples.
• Added executable scripts/commit-msg.sh to validate the first real subject line, allow merge/revert commits, require an allowed Conventional Commit type with optional scope, require a non-empty description, and enforce the 72-character subject limit.
• Updated make install-hooks to install both pre-commit and commit-msg hooks.
• Documented the hook behavior and git config commit.template .gitmessage setup in README.

Verification

• Manual commit-msg hook checks with temporary valid, invalid, merge, and revert message files.
• make install-hooks
• go test ./...
• Commit verified through installed pre-commit and commit-msg hooks.