fix: proposal threads disable the reply input — decide-only, INV-12

Playing with F4, the operator hit VS Code's default "Reply…" box on a
proposal thread: no submit command is wired for cowriting.proposals, so
typed text had nowhere to go. Proposals are decide-only (accept/reject in
the title bar); discussion belongs in a coauthoring thread. Set
canReply=false so the dead input never renders, and expose canReply on the
test-facing RenderedProposal with an E2E assertion.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/proposalController.ts

@@ -22,6 +22,8 @@ export interface RenderedProposal {
   id: string;
   /** true → resolves exactly, decidable; false → stale/orphaned (INV-11). */
   pending: boolean;
+  /** always false — proposals are decide-only, no reply input (INV-12). */
+  canReply: boolean;
   turnId?: string;
   range: { start: number; end: number };
 }
@@ -252,6 +254,11 @@ export class ProposalController implements vscode.Disposable {
         author: { name: proposal.author.id },
       },
     ]);
+    // Proposals are decide-only (INV-12): ✓ accept / ✗ reject in the title
+    // bar. Without this, VS Code renders its default "Reply…" input with no
+    // submit command wired — a dead end. Discussion belongs in a regular
+    // coauthoring thread; proposal discussion trails are deferred (spec §1.7).
+    vsThread.canReply = false;
     vsThread.label = pending
       ? "Pending proposal"
       : "⚠ Stale proposal (target text changed or missing) — accept disabled";
@@ -315,12 +322,13 @@ export class ProposalController implements vscode.Disposable {
     const state = this.docs.get(docPath);
     if (!state) return [];
     const out: RenderedProposal[] = [];
-    for (const [id] of state.vsThreads) {
+    for (const [id, vsThread] of state.vsThreads) {
       const p = state.artifact.proposals.find((x) => x.id === id)!;
       const off = state.live.get(id)!;
       out.push({
         id,
         pending: !state.unresolved.has(id),
+        canReply: vsThread.canReply !== false,
         turnId: p.turnId,
         range: { start: off.start, end: off.end },
       });