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>

test/e2e/suite/proposals.test.ts

@@ -74,6 +74,7 @@ suite("F4 propose/accept (host E2E — programmatic ingress, no LLM)", () => {
     assert.strictEqual(rendered.length, 1);
     assert.strictEqual(rendered[0].pending, true);
     assert.strictEqual(rendered[0].turnId, "turn-p1");
+    assert.strictEqual(rendered[0].canReply, false, "proposals are decide-only — no dead reply input (INV-12)");
     const art = readSidecar();
     assert.strictEqual(art.proposals.length, 1, "proposal persisted at propose time");
     assert.strictEqual(art.anchors[art.proposals[0].anchorId].fingerprint.text, TARGET);