Release 0.5.0: PR-less per-RFC discussion (contribution still requires PR)

Roadmap item #3. An RFC's main view now carries a discussion surface
distinct from PR comments and from branch chat. The substrate is the
existing threads/thread_messages tables — rows with branch_name IS NULL
scope to the RFC's main view; the schema already permitted that shape,
v0.5.0 is the first build to write it. Five new endpoints under
/api/rfcs/<slug>/discussion/..., a new RFCDiscussionPanel right-column
component used when branchParam === main, SPEC §10.10 settling
discussion-vs-contribution, and §17 listing the new routes. Notification
routing reuses the existing chat_message_in_participated_thread /
chat_reply_to_my_message event kinds with branch_name=null on the
fan-out row; a distinct event_kind is a §19.2 candidate. Anonymous
viewers can read; writes require contributor — v0.6.0's item #4 will
harden adjacent gates. No schema migration; minor bump, no operator
action required beyond rebuild and restart.

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

frontend/src/api.js

@@ -197,6 +197,52 @@ export async function resolveThread(slug, branch, threadId) {
   return jsonOrThrow(res)
 }
 
+// ── v0.5.0: PR-less per-RFC discussion (§5 / §10) ────────────────────────
+//
+// The substrate is `threads.branch_name IS NULL` — the same threads
+// table the branch chat uses, with a null branch the schema already
+// supported. Contribution still requires a PR (api_prs / openPR), so
+// these endpoints are read+write for discussion only.
+
+export async function listDiscussionThreads(slug) {
+  return jsonOrThrow(await fetch(`/api/rfcs/${slug}/discussion/threads`))
+}
+
+export async function createDiscussionThread(slug, { label = null, message = null } = {}) {
+  const res = await fetch(`/api/rfcs/${slug}/discussion/threads`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ label, message }),
+  })
+  return jsonOrThrow(res)
+}
+
+export async function getDiscussionThreadMessages(slug, threadId) {
+  return jsonOrThrow(await fetch(
+    `/api/rfcs/${slug}/discussion/threads/${threadId}/messages`,
+  ))
+}
+
+export async function postDiscussionMessage(slug, threadId, { text, quote = null }) {
+  const res = await fetch(
+    `/api/rfcs/${slug}/discussion/threads/${threadId}/messages`,
+    {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ text, quote }),
+    },
+  )
+  return jsonOrThrow(res)
+}
+
+export async function resolveDiscussionThread(slug, threadId) {
+  const res = await fetch(
+    `/api/rfcs/${slug}/discussion/threads/${threadId}/resolve`,
+    { method: 'POST' },
+  )
+  return jsonOrThrow(res)
+}
+
 // ── Slice 4: super-draft body editing (§9.5) ─────────────────────────────
 
 export async function startEditBranch(slug, body = {}) {