docs(agents.md): require Conventional Commits for commit messages and PR titles

[minixalpha]

Summary

Add constraints to agents.md so that any agent (Claude Code, Codex, pi, etc.) working in this repo follows the Conventional Commits 1.0.0 spec for commit messages and PR titles.

Note: agents.md does not exist in the repo yet. This issue covers either creating it or adding the relevant section once it exists.

Motivation

• Consistent, machine-readable commit history and PR titles.
• Enables automated changelog generation and semantic versioning down the line.
• Gives agents an explicit, shared convention to follow instead of ad-hoc messages.

Proposed constraints

Document the following in agents.md:

• Commit messages MUST follow Conventional Commits: <type>[optional scope][!]: <description>.
• PR titles MUST follow the same format (since squash-merge often uses the PR title as the commit subject).
• Allowed types: feat, fix, docs, style, refactor, perf, test, build, ci, chore, revert.
• Breaking changes: use ! after the type/scope and/or a BREAKING CHANGE: footer.
• Keep the description in the imperative mood, lowercase, no trailing period.
• Optional body and footers separated by a blank line.

Reference

• https://www.conventionalcommits.org/en/v1.0.0/