feat(hooks): add commit-msg hook to normalize commit messages

[ke23y]

Summary

• Adds scripts/commit-msg.sh, a Conventional Commits validator for the commit subject line, matching the existing scripts/pre-commit.sh style (set -euo pipefail, ✓/✗ output, exit 1 on failure, --no-verify escape hatch).
• Wires it into make install-hooks alongside the existing pre-commit hook, and updates the help text.
• Documents the convention (<type>(<scope>)?: <description>, allowed types) in AGENTS.md under Conventions.
• Merge and revert commits are allowed through untouched so normal GitHub merge workflows aren't blocked.

Test plan

• bash -n scripts/commit-msg.sh
• Ran the hook against a conforming subject (fix(budget): correct rollover calc) — passes
• Ran the hook against a non-conforming subject (Fixed the budget thing) — fails with a clear message
• Ran the hook against a merge commit subject — passes
• make install-hooks installs both .git/hooks/pre-commit and .git/hooks/commit-msg symlinks
• go build ./... and go vet ./... pass (no Go code touched)
• This PR's own commit was made with both hooks installed and passed both checks

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