docs(agents): tighten provider boundary guidance

extensions/AGENTS.md

@@ -36,6 +36,21 @@ third-party plugins see.
   matching `src/plugin-sdk/<id>.ts` facade.
 - If core or core tests need a bundled plugin helper, export it from `api.ts`
   first instead of letting them deep-import extension internals.
+- For provider plugins, keep auth, onboarding, catalog selection, and
+  vendor-only product behavior local to the plugin. Do not move those into
+  core just because two providers look similar.
+- Before adding a new provider-local `wrapStreamFn`, `buildReplayPolicy`,
+  `normalizeToolSchemas`, `inspectToolSchemas`, or compat patch helper, check
+  whether the same behavior already exists through `openclaw/plugin-sdk/*`.
+  Reuse shared family helpers first.
+- If two bundled providers share the same replay policy shape, tool-schema
+  compat rewrite, payload patch, or stream-wrapper chain, stop copying the
+  logic. Extract one shared helper and migrate both call sites in the same
+  change.
+- Prefer named provider-family helpers over repeating raw option bags. If a
+  provider needs OpenAI-style Anthropic tool payload compat, Gemini schema
+  cleanup, or an XAI compat patch, use a named shared helper instead of
+  inlining the policy knobs again.
 
 ## Expanding The Boundary