Add agent operating guide

AGENTS.md

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+# AGENTS.md
+
+- Treat matcha/notplanned as canonical; fetch/reconcile main before writes.
+- Prefer deterministic GitHub/API scans and existing workflows over model calls.
+- Keep scouts bounded, inspectable, and cheap. Raw evidence beats narrative.
+- Preserve existing visual/product language; make the smallest change that adds a new signal.
+- Verify generated artifacts and the live route before DONE.