docs: codify commit message and code comment conventions

[imantsk]

Adds two sections to AGENTS.md (the single source of truth all tool shims point to):

• Commit Messages — single-line conventional commits starting with fix:/feat:/chore:/docs: only, no scope parentheses, no co-author or other trailers, one concern per commit.
• Code Comments — comments state current constraints the code cannot show; no narration, no change-justification, no arguing with previous implementations — history belongs in the commit message.

Codifies conventions already practised in review so all agents (Claude Code, Codex, Cursor, Copilot) inherit them from one place.