fix: provider-aware model routing with fallback visibility

- Extract createClientFromConfig() to dispatch on provider field instead
  of hardcoding all tiers as AnthropicClient
- Add fallback/fallbackReason metadata to ChatResponse and ChatStreamEvent
  so callers know when a fallback model was used
- Enhance doctor check to report full model stack and warn on missing
  API keys for cloud providers
- Log fallback warnings in NativeAgent and display them in TUI
- Support tier names and local_providers entries in fallback_chain
- Add 8 tests for createClientFromConfig covering all provider types

src/cli/doctor.test.ts

@@ -0,0 +1,125 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { runChecks, type CheckResult, type DoctorContext } from './doctor.js';
+import { writeFileSync, mkdirSync, rmSync } from 'fs';
+import { join } from 'path';
+import { tmpdir } from 'os';
+
+describe('doctor checks', () => {
+  const testDir = join(tmpdir(), 'flynn-test-doctor');
+
+  afterEach(() => {
+    try { rmSync(testDir, { recursive: true }); } catch {}
+  });
+
+  it('reports PASS when config file exists and is valid', async () => {
+    mkdirSync(testDir, { recursive: true });
+    const configPath = join(testDir, 'config.yaml');
+    writeFileSync(configPath, `
+telegram:
+  bot_token: "test-token"
+  allowed_chat_ids: [123]
+models:
+  default:
+    provider: anthropic
+    model: claude-sonnet
+`);
+
+    const ctx: DoctorContext = { configPath, dataDir: testDir };
+    const results = await runChecks(ctx);
+
+    const configExists = results.find(r => r.label.includes('Config file'));
+    expect(configExists?.status).toBe('pass');
+
+    const configParses = results.find(r => r.label.includes('parses'));
+    expect(configParses?.status).toBe('pass');
+
+    const configValidates = results.find(r => r.label.includes('validates'));
+    expect(configValidates?.status).toBe('pass');
+  });
+
+  it('reports FAIL when config file does not exist', async () => {
+    const ctx: DoctorContext = { configPath: '/nonexistent/config.yaml', dataDir: testDir };
+    const results = await runChecks(ctx);
+
+    const configExists = results.find(r => r.label.includes('Config file'));
+    expect(configExists?.status).toBe('fail');
+  });
+
+  it('reports FAIL on invalid YAML', async () => {
+    mkdirSync(testDir, { recursive: true });
+    const configPath = join(testDir, 'bad.yaml');
+    writeFileSync(configPath, '{{{{bad yaml');
+
+    const ctx: DoctorContext = { configPath, dataDir: testDir };
+    const results = await runChecks(ctx);
+
+    const configParses = results.find(r => r.label.includes('parses'));
+    expect(configParses?.status).toBe('fail');
+  });
+
+  it('reports FAIL on schema validation failure', async () => {
+    mkdirSync(testDir, { recursive: true });
+    const configPath = join(testDir, 'invalid.yaml');
+    writeFileSync(configPath, `
+telegram:
+  bot_token: ""
+`);
+
+    const ctx: DoctorContext = { configPath, dataDir: testDir };
+    const results = await runChecks(ctx);
+
+    const configValidates = results.find(r => r.label.includes('validates'));
+    expect(configValidates?.status).toBe('fail');
+  });
+
+  it('reports PASS for writable data directory', async () => {
+    mkdirSync(testDir, { recursive: true });
+    const configPath = join(testDir, 'config.yaml');
+    writeFileSync(configPath, `
+telegram:
+  bot_token: "test-token"
+  allowed_chat_ids: [123]
+models:
+  default:
+    provider: anthropic
+    model: claude-sonnet
+`);
+
+    const ctx: DoctorContext = { configPath, dataDir: testDir };
+    const results = await runChecks(ctx);
+
+    const dataDir = results.find(r => r.label.includes('Data directory'));
+    expect(dataDir?.status).toBe('pass');
+  });
+
+  it('reports PASS for accessible session DB', async () => {
+    mkdirSync(testDir, { recursive: true });
+    const configPath = join(testDir, 'config.yaml');
+    writeFileSync(configPath, `
+telegram:
+  bot_token: "test-token"
+  allowed_chat_ids: [123]
+models:
+  default:
+    provider: anthropic
+    model: claude-sonnet
+`);
+
+    const ctx: DoctorContext = { configPath, dataDir: testDir };
+    const results = await runChecks(ctx);
+
+    const sessionDb = results.find(r => r.label.includes('Session DB'));
+    expect(sessionDb?.status).toBe('pass');
+  });
+
+  it('skips downstream checks when config is invalid', async () => {
+    const ctx: DoctorContext = { configPath: '/nonexistent/config.yaml', dataDir: testDir };
+    const results = await runChecks(ctx);
+
+    const modelCheck = results.find(r => r.label.includes('Model connectivity'));
+    expect(modelCheck?.status).toBe('skip');
+
+    const telegramCheck = results.find(r => r.label.includes('Telegram'));
+    expect(telegramCheck?.status).toBe('skip');
+  });
+});