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

.githooks/pre-commit

@@ -1,15 +1,9 @@
 #!/usr/bin/env bash
 set -euo pipefail
 changed=$(git diff --cached --name-only)
-# fail if REQs changed but AGENTS.md is older
-if echo "$changed" | grep -q '^REQUIREMENTS.md$' && echo "$changed" | grep -q '^AGENTS.md$'; then
-  # Both changed — OK
-  :
-else
-  req_time=$(git log -1 --format=%ct -- REQUIREMENTS.md 2>/dev/null || echo 0)
-  ag_time=$(git log -1 --format=%ct -- AGENTS.md 2>/dev/null || echo 0)
-  if [ "$req_time" -gt "$ag_time" ]; then
-    echo "error: REQUIREMENTS.md newer than AGENTS.md — run: redgate agents > AGENTS.md"
-    exit 1
-  fi
+# fail only when this commit changes REQs without reviewing AGENTS.md;
+# do not block unrelated commits just because an earlier commit changed REQs.
+if echo "$changed" | grep -q '^REQUIREMENTS.md$' && ! echo "$changed" | grep -q '^AGENTS.md$'; then
+  echo "error: REQUIREMENTS.md changed without AGENTS.md — review AGENTS.md or run: redgate agents > AGENTS.md"
+  exit 1
 fi