fix(native-chat): degrade instead of throwing on a null tab title

A stacked branch can publish title: null when a conversation name is
cleared. The wire type says string, so this consumer trusted it and
threw inside the store patch that applies the snapshot. Fall back to the
placeholder — the producer bug is fixed separately, but a consumer of
wire data should not crash on a contract violation.

src/renderer/src/runtime/web-session-tabs-sync/mirrored-agent-tab-label.test.ts

@@ -67,6 +67,14 @@ describe('buildMirroredAgentTabs', () => {
     expect(tab.customLabel).toBeNull()
   })
 
+  it('degrades to the placeholder when the host violates the string contract', () => {
+    const snapshot = snapshotWith('claude', 'Named')
+    // The wire type says `string`, but a host clearing a name can send null.
+    ;(snapshot.tabs[0] as { title: unknown }).title = null
+    expect(() => build(snapshot)).not.toThrow()
+    expect(build(snapshot).label).toBe('Claude Chat')
+  })
+
   it('names an agent this build does not know after itself, not Codex', () => {
     const snapshot = snapshotWith('codex', '')
     // Cast: the wire union is claude|codex today, but Tab.agentSessionAgent is

src/renderer/src/runtime/web-session-tabs-sync/terminal-surfaces.ts

@@ -74,7 +74,9 @@ export function buildMirroredAgentTabs(
         worktreeId: snapshot.worktree,
         contentType: 'agent-session',
         agentSessionAgent: tab.agent,
-        label: tab.title.trim() || defaultAgentChatLabel(tab.agent),
+        // Why: `title` is wire data typed `string`; a host that violates that must
+        // degrade to the placeholder, not throw inside the snapshot patch.
+        label: tab.title?.trim() || defaultAgentChatLabel(tab.agent),
         // Why: a manual rename lives only on the client; re-nulling it here made
         // every host snapshot silently discard the user's title.
         customLabel: existing?.customLabel ?? null,