Normalize commit messages with a commit-msg hook

[marcus]

Summary

• add a commit-msg hook that rewrites human-authored subjects to type: summary (td-<id>)
• install the new hook alongside pre-commit and add shell coverage for normalization/rejection behavior
• update contributor, agent, and release docs to use the canonical format and document the td: bump to vX.Y.Z automation exception

Verification

• bash ./scripts/test_commit-msg.sh
• make install-hooks
• go test ./... (fails in pre-existing internal/features tests: TestKnownFeatureDefaults, TestIsEnabledForProcess_EnableDisableLists, TestIsEnabled_ProjectConfigAndEnvPrecedence)
• make test (fails for the same pre-existing internal/features tests)