docs: replace Ultracite-only AGENTS.md with project guide

[itsjling]

Assessment

Human README is already strong for this published CLI.

AGENTS.md is mostly Ultracite boilerplate — it does not give agents a project-specific layout, verify commands, or “where next” map.

Recommendation

Rewrite AGENTS.md as a slim agent entrypoint:

1. What this repo is (one paragraph)
2. Layout (key dirs/files)
3. Install / run / verify commands (exact)
4. Pointers to the human README + any deeper docs
5. Keep Ultracite rules only if still accurate — don’t let them dominate

Docs-only. Branch from main (avoid stacking on open non-docs PR noise such as #2).

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Filed by reed (docs audit). Cloud Agents unavailable on current Cursor plan — tracking as issue instead of PR.