📝 Add AGENTS.md as the agent entry point

AI coding agents read AGENTS.md automatically, not README. Make AGENTS.md
a thin pointer: the stable first-action facts plus links to CONTRIBUTING.md,
README.md, and package.json#scripts. Keep the rules prose in CONTRIBUTING.md
so humans and agents don't diverge (same anti-drift move as dropping
project-specs.md).

Clarify the agent's verification loop: npm run test is the mandatory gate;
the pre-commit hook already runs the fast offline checks (tsc, oxlint, oxfmt,
cspell) on staged files, and npm run check (knip + outdated) is repo-maintenance,
not part of the feature loop. Forbid type-system escape hatches (@ts-nocheck,
oxlint-disable, as-casts) as agent-only rules; a human may still add a
review-visible disable as a last resort, so the location convention stays in
CONTRIBUTING.md with a forward reference.

README.md

@@ -66,7 +66,7 @@ Script names follow a prefix convention that signals _when_ they run:
 
 ## Contributing
 
-For maintainer and contributor docs — the script prefix convention, the feedback-tier system, and the publishing workflow — see [CONTRIBUTING.md](./CONTRIBUTING.md).
+For maintainer and contributor docs — the script prefix convention, the feedback-tier system, the rules the tools don't enforce, and the publishing workflow — see [CONTRIBUTING.md](./CONTRIBUTING.md). AI coding agents: your entry point is [AGENTS.md](./AGENTS.md), which points back to CONTRIBUTING.md.
 
 - Commit signing (GPG).
 - Set up commit message template: `npm run use:git-commit-message`.