Slice 4: super-draft body editing per §9.5 + §9.6

The §17 routing-collapse rule lands in api_branches.py and
api_prs.py — every branches/<branch>/... and prs/<n>/... route
dispatches on the entry's state to pick the right Gitea repo, and
the body extracted from the entry's frontmatter envelope is what
the editor and the diff see. The bot grows open_metadata_pr;
cache grows refresh_meta_branches. Two §17 routes added:
start-edit-branch and metadata. The §9.4 super-draft view replaces
RFCView.jsx's Slice 2 placeholder; a metadata pane modal opens
from the breadcrumb. Branch naming uses edit-<slug>-<6hex> to
dodge the §19.2 path-routing candidate while preserving §9.5's
structural shape.

Covered by tests/test_super_draft_vertical.py (10 tests). The
full Slices 1-4 suite is 35/35 green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/api.js

@@ -197,6 +197,30 @@ export async function resolveThread(slug, branch, threadId) {
   return jsonOrThrow(res)
 }
 
+// ── Slice 4: super-draft body editing (§9.5) ─────────────────────────────
+
+export async function startEditBranch(slug, body = {}) {
+  const res = await fetch(`/api/rfcs/${slug}/start-edit-branch`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body),
+  })
+  return jsonOrThrow(res)
+}
+
+export async function editMetadata(slug, { title, tags, prDescription }) {
+  const res = await fetch(`/api/rfcs/${slug}/metadata`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({
+      title: title ?? null,
+      tags: tags ?? null,
+      pr_description: prDescription ?? null,
+    }),
+  })
+  return jsonOrThrow(res)
+}
+
 // ── Slice 3: the §10 PR flow ─────────────────────────────────────────────
 
 export async function draftPRText(slug, branch) {