docs(boundary): codify shared test helper plugin seams

test/helpers/AGENTS.md

@@ -0,0 +1,31 @@
+# Shared Test Helper Boundary
+
+This directory holds shared test helpers reused by core and bundled plugin
+tests.
+
+## Bundled Plugin Imports
+
+- Shared helpers in this tree must not hardcode repo-relative imports into
+  `extensions/**`.
+- When a helper needs a bundled plugin public surface, go through
+  `src/test-utils/bundled-plugin-public-surface.ts`.
+- Prefer `loadBundledPluginApiSync(...)`,
+  `loadBundledPluginRuntimeApiSync(...)`,
+  `loadBundledPluginContractApiSync(...)`, and
+  `loadBundledPluginTestApiSync(...)` for eager access to exported surfaces.
+- Prefer `resolveRelativeBundledPluginPublicModuleId(...)` or
+  `resolveBundledPluginPublicModulePath(...)` when a helper needs a module id
+  or filesystem path for dynamic import, mocking, or loading a plugin entrypoint
+  such as `index.js`.
+- If `vi.hoisted(...)` is involved, do not call imported helper functions from
+  inside the hoisted callback. Resolve the module id outside the callback or
+  switch to `vi.doMock(...)`.
+- Do not keep plugin-local deep mocks or private `src/**` knowledge in shared
+  helpers. Move those helpers into the owning bundled plugin package instead.
+
+## Intent
+
+- Keep shared helpers aligned with the same public/plugin boundary that
+  production code uses.
+- Avoid shared helper debt that makes core test lanes depend on bundled plugin
+  private layout.

test/helpers/CLAUDE.md

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+AGENTS.md

test/helpers/channels/AGENTS.md

@@ -3,6 +3,8 @@
 This directory holds shared channel test helpers used by core and bundled plugin
 tests.
 
+This file adds channel-specific rules on top of `test/helpers/AGENTS.md`.
+
 ## Bundled Plugin Imports
 
 - Core test helpers in this directory must not hardcode repo-relative imports