docs: add agent rules for documentation standards

AGENT.md

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+# Agent Rules
+
+This file defines high-priority implementation and documentation rules for coding agents.
+
+## Always Apply
+
+1. Prefer minimal, reversible changes.
+2. Preserve behavior unless explicitly requested to change it.
+3. Do not claim verification succeeded unless commands were run.
+4. Report exact verification commands and pass/fail outcomes.
+
+## Documentation Rules
+
+For changes in `src/`, apply these rules to exported/public APIs:
+
+1. Use TSDoc (`/** ... */`) with a short behavior-focused summary.
+2. Add explicit `@param` tags for each parameter.
+3. Add explicit `@returns` tags.
+4. Add `@throws` for intentional thrown errors.
+5. Update docs in the same change as the behavior change.
+6. Avoid repeating obvious implementation detail.
+
+## Testing Rules
+
+1. Add or update tests in the same change when behavior changes.
+2. Run targeted tests first, then broader suites as needed.
+3. Keep test naming in `<method> <conditions> <expected>` format.
+
+## Minimum Verification
+
+```bash
+npm run typecheck
+npm exec vitest run test/action.test.ts test/action-runner.test.ts test/manifest.test.ts test/io.test.ts
+```
+
+When changing scripts:
+
+```bash
+npm exec vitest run test/dev_scripts.test.ts
+```