docs: explain reusable partials

Add the public 'Where are my components?' answer: hemx maps framework component jobs to hemplate partials, generated helpers, app-owned state, IntoEffect composition, and explicit leaf islands. The pre-commit guard now only blocks commits that change REQUIREMENTS.md without an AGENTS.md review, so unrelated follow-up commits are not trapped by old history.

req: canonical_authoring/002

req: canonical_authoring/003

README.md

@@ -42,6 +42,10 @@ For beginner and production-shaped app code, stay on this path. req: public_api/
 - **Effect:** handlers return typed commands that become a checked effect
   response. Tuple composition is the normal fixed batch syntax; arrays and
   `Vec<T: IntoEffect>` cover fixed or dynamic repeated partial updates.
+- **Reuse:** the hemx answer to framework components is reusable hemplate
+  partials plus generated helpers, app-owned state, `IntoEffect` composition,
+  and explicit leaf islands when browser-owned behavior is necessary. See
+  `docs/recipes/reusable-partials.md`.
 - **Runtime:** the browser checks the build fingerprint, resolves targets within
   the current `data-hemx-root`, and applies compatible batches. Mismatched
   server/runtime builds fail closed instead of silently mutating the wrong DOM.