wip: context-meter + model-label UI and provider/inference tweaks

Checkpoint of in-flight work so the orchestrator branch can rebase onto a
clean main: ContextBar → ContextMeter, model-label helper, model/agent picker
+ provider-snapshot/registry changes, inference payload + message-columns.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/services/__tests__/inference.test.ts

@@ -111,6 +111,42 @@ describe('buildMessagesPayload', async () => {
     expect(result[4]).toMatchObject({ role: 'assistant', content: 'great' });
   });
 
+  it('does NOT annotate models when the chat uses a single model', async () => {
+    const session = makeSession();
+    const project = makeProject();
+    const history: Message[] = [
+      makeMessage('user', 'hi'),
+      makeMessage('assistant', 'hello', { model: 'qwen3.6-35b-a3b-mxfp4' }),
+      makeMessage('user', 'again'),
+      makeMessage('assistant', 'world', { model: 'qwen3.6-35b-a3b-mxfp4' }),
+    ];
+    const result = await buildMessagesPayload(session, project, history);
+    // 1 system + 4 history — no extra attribution system note.
+    expect(result).toHaveLength(5);
+    expect(result.filter((m) => m.role === 'system')).toHaveLength(1);
+    expect(result[2]).toMatchObject({ role: 'assistant', content: 'hello' });
+    expect(result[4]).toMatchObject({ role: 'assistant', content: 'world' });
+  });
+
+  it('annotates each assistant turn with its model when the chat mixes models', async () => {
+    const session = makeSession();
+    const project = makeProject();
+    const history: Message[] = [
+      makeMessage('user', 'hi'),
+      makeMessage('assistant', 'opus reply', { model: 'claude-opus-4-8' }),
+      makeMessage('user', 'switch'),
+      makeMessage('assistant', 'qwen reply', { model: 'qwen3.6-35b-a3b-mxfp4' }),
+    ];
+    const result = await buildMessagesPayload(session, project, history);
+    // 1 system prompt + 1 attribution note + 4 history rows.
+    const systems = result.filter((m) => m.role === 'system');
+    expect(systems).toHaveLength(2);
+    expect(systems[1]!.content).toContain('square brackets');
+    const assistants = result.filter((m) => m.role === 'assistant');
+    expect(assistants[0]!.content).toBe('[claude-opus-4-8] opus reply');
+    expect(assistants[1]!.content).toBe('[qwen3.6-35b-a3b-mxfp4] qwen reply');
+  });
+
   it('starts from the latest compact marker, emitting it as a system message', async () => {
     const session = makeSession();
     const project = makeProject();