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/components/RFCView.jsx

@@ -39,6 +39,7 @@ import MarkdownPreview from './MarkdownPreview.jsx'
 import SelectionTooltip from './SelectionTooltip.jsx'
 import PromptBar from './PromptBar.jsx'
 import ChatPanel from './ChatPanel.jsx'
+import RFCDiscussionPanel from './RFCDiscussionPanel.jsx'
 import ChangePanel, { diffWords } from './ChangePanel.jsx'
 import PRModal from './PRModal.jsx'
 import GraduateDialog from './GraduateDialog.jsx'
@@ -759,17 +760,25 @@ export default function RFCView({ viewer }) {
           data-open={drawerOpen ? 'true' : 'false'}
         />
         <div className={`right-panel${drawerOpen ? ' drawer-open' : ''}`} role="complementary">
-          <ChatPanel
-            messages={messages}
-            threads={branchView.threads || []}
-            changes={changes}
-            branchName={branchParam}
-            isStreaming={isStreaming}
-            contributionMode={mode === 'contribute'}
-            onStartContribution={handleStartContributing}
-            onScrollToChange={setFocusedChangeId}
-            onResolveThread={handleResolveThread}
-          />
+          {/* v0.5.0 — on main, the right panel is the PR-less discussion
+            * surface (threads.branch_name IS NULL). Branches keep their
+            * existing branch-chat panel; contribution still requires
+            * opening a PR from a branch via the Open PR affordance above. */}
+          {branchParam === 'main' ? (
+            <RFCDiscussionPanel slug={slug} viewer={viewer} />
+          ) : (
+            <ChatPanel
+              messages={messages}
+              threads={branchView.threads || []}
+              changes={changes}
+              branchName={branchParam}
+              isStreaming={isStreaming}
+              contributionMode={mode === 'contribute'}
+              onStartContribution={handleStartContributing}
+              onScrollToChange={setFocusedChangeId}
+              onResolveThread={handleResolveThread}
+            />
+          )}
           {mode === 'contribute' && (changes.length > 0 || manualPending) && (
             <ChangePanel
               changes={changes}