Docs/agent guidelines (#6129)

* docs: refresh agent guidelines

* docs: add agent harness adapters

* Update local-only block markers in AGENTS.md

DO NOT COMMIT phrase was confusing most LLMs and preventing them from committing anything without an additional explicit permission.

.github/copilot-instructions.md

@@ -0,0 +1,6 @@
+# Agent-guide adapter
+
+Before starting any repository task, open and follow the repository-root `AGENTS.md`.
+That guide defines the scoped `AGENTS.md` files that must also be opened before editing
+their paths. Treat the root and applicable scoped guides as mandatory. Do not duplicate
+or replace their rules in this file.