docs: add project-level AGENTS.md as the shared agent/contributor guide
Promote the agent guidance that was previously a personal, gitignored root file
into a committed, contributor-facing AGENTS.md, and tie together the existing
.agents/ resources (architecture invariants, generated-files list, skills) and
the per-crate AGENTS.md guides under one auto-discoverable root entry point.
- Add root AGENTS.md: core commands, repo map, must-read rules, high-signal
entry points, and pre-PR checks. Worded for any coding agent, not just one.
- CLAUDE.md becomes a symlink to AGENTS.md for Claude Code compatibility.
- .gitignore: stop ignoring root AGENTS.md/CLAUDE.md; ignore /.local/ instead,
which is where personal/local agent config now lives.
Signed-off-by: Dennis Zhuang <killme2008@gmail.com>
