Add AGENTS.md with the agent-facing tsk crib sheet
A short, operational reference for agents working in this repo: the
commands they'll actually use, the namespace/queue convention (tsk vs
claude), how to record TODOs in the claude queue without polluting the
user's stack, and the markup grammar for tsk show.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>