§22 S2: collection-scoped frontend path + API helpers

useCollectionId() hook; listRFCs/getRFC/proposeRFC take an optional collection
id and target the /collections/<cid>/ routes; add listCollections +
createCollection.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/entryPaths.js

@@ -4,6 +4,7 @@
 // project-scoped, so the builders emit the default collection segment; the
 // collection-aware link layer (named collections) lands in S2. Components build
 // links via these helpers so that flip happens in one place.
+import { useParams } from 'react-router-dom'
 import { useProject } from '../components/ProjectLayout.jsx'
 import { useDeployment } from '../context/DeploymentProvider'
 
@@ -37,3 +38,10 @@ export function useProjectId() {
   const { defaultProjectId } = useDeployment()
   return (ctx && ctx.projectId) || defaultProjectId
 }
+
+// §22 S2 — the collection id a component should scope to: the `/c/:collectionId/`
+// route segment when present, else the project's default collection.
+export function useCollectionId() {
+  const { collectionId } = useParams()
+  return collectionId || DEFAULT_COLLECTION
+}