Stream C frontend: ChatCoach page, nav update, README with AI providers + agent configs

Frontend:
- ChatCoach.jsx: streaming chat UI with SSE, provider indicator, suggestion
  chips, 'no AI configured' state with setup link
- App.jsx: import + /chat route + nav bar (More → Coach with brain emoji)
- api.js: getAIStatus() method

Documentation:
- README.md: 'Built-in AI Coach' section with 8-provider table, external
  agent config snippets for Claude Desktop, Claude Code, Cursor, Windsurf,
  and COROS multi-MCP pattern
- AGENT_GUIDE.md: multi-device integration, Strava AI/ML warning,
  provider-agnostic statement

Nav bar: Home | Log | Keto | Programs | Coach
Settings still reachable at /settings (linked from Coach setup prompt)

frontend/src/api.js

@@ -139,6 +139,10 @@ export const api = {
   listProviders: () => request('/integrations/providers'),
   configureIntegration: (provider, credentials) =>
     request('/integrations/configure', { method: 'POST', body: JSON.stringify({ provider, credentials }) }),
+  // AI Coaching
+  getAIStatus: () => request('/ai/status'),
+  // Note: chatWithAI uses fetch directly in ChatCoach.jsx for SSE streaming
+
   // Resources
   getResourceRecommendations: () => request('/onboarding/recommendations'),
 };