fix(acp): wait for providers to load before accepting requests

[chaodu-agent]

Summary

After DB migration, the SDK server may not have fully indexed providers before the first ACP request arrives. Add a readiness check before accepting connections.

Current Behavior

┌─── mimo acp cold start ─────────────────────────────────────────┐
│                                                                  │
│  t0  DB migration runs (rewrites SQLite tables)                  │
│  t1  migration done                                              │
│  t2  Server.listen()                                             │
│  t3  ACP agent created ← IMMEDIATELY accepts stdin               │
│  t4  1st request arrives                                         │
│       ├─ sdk.config.providers()                                  │
│       │   └─ [xiaomi] ← mimo provider NOT YET INDEXED   ❌       │
│       ├─ defaultModel() → Provider.sort()                        │
│       │   └─ picks mimo-v2.5-pro-ultraspeed (paid)      ❌       │
│       └─ 0 tokens → "(no response)"                             │
│                                                                  │
│  t5  2nd request → providers fully loaded → works        ✅      │
│                                                                  │
└──────────────────────────────────────────────────────────────────┘

Expected Behavior

┌─── mimo acp cold start (with fix) ──────────────────────────────┐
│                                                                  │
│  t0  DB migration runs                                           │
│  t1  migration done                                              │
│  t2  Server.listen()                                             │
│  t3  readiness loop: poll providers every 500ms (max 5s)         │
│       └─ waits until providers.length > 0                        │
│  t4  providers loaded → ACP agent created → accepts stdin        │
│  t5  1st request arrives                                         │
│       ├─ sdk.config.providers()                                  │
│       │   └─ [mimo, xiaomi, ...]                         ✅      │
│       ├─ defaultModel() → mimo/mimo-auto                 ✅      │
│       └─ works on first try!                                     │
│                                                                  │
└──────────────────────────────────────────────────────────────────┘

Relationship to #865

Belt-and-suspenders with PR #1170:

• fix(acp): honor config.json model when provider not yet loaded #1170: if provider missing, still honor user config (defensive)
• fix(acp): wait for providers to load before accepting requests #1171: ensure provider IS loaded before accepting requests (proactive)

Fixes #866

[chaodu-agent]

Superseded by #1172 (combined with #1170, added unit tests)