Docs: capture test performance guardrails

test/helpers/channels/AGENTS.md

@@ -18,6 +18,13 @@ This file adds channel-specific rules on top of `test/helpers/AGENTS.md`.
 - If `vi.mock(...)` hoisting would evaluate the module id too early, use
   `vi.doMock(...)` with the resolved module id instead of falling back to a
   hardcoded path.
+- For contract helpers, prefer minimal in-memory channel/plugin fixtures when
+  the contract only needs capabilities, session binding hooks, routing metadata,
+  or outbound payload helpers. Do not load broad `api.ts`, `runtime-api.ts`, or
+  `test-api.ts` barrels for incidental setup.
+- If a bundled plugin parser is the contract under test, load the narrow module
+  that owns that parser or promote a small public artifact. Avoid pulling a full
+  extension barrel just to parse a target id.
 
 ## Intent