docs(agents): add Skills instruction SSOT notes

AGENTS.md

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+# brand — local agent notes only
+
+Static engineering and delivery standards load from the active Skills runtime
+([SylphxAI/skills](https://github.com/SylphxAI/skills) is binding instruction
+SSOT). Doctrine and Mission Control are retired historical lineage and must not
+be loaded as current instruction authority.
+
+Local truth: `PROJECT.md` when present.
+
+## Boundary hazards
+
+- Never commit secrets, tokens, `.env` files, or credentials.
+
+## Validation notes
+
+- Prefer the narrowest affected check before full workspace runs.
+- Report layers honestly: local · CI · merge · deploy · live proof.