feat(agy): Persistent single-process driver, stream-json protocol, auto-retry & dual-engine architecture

[prophetw]

Summary

This PR overhauls the Antigravity (`agy`) adapter in Happy CLI, upgrading it from a per-turn spawned CLI runner to a **native, long-lived persistent single-process architecture** with full **Stream-JSON** protocol support, **automatic network error

recovery**, session title extraction, and a dual-engine backend (Persistent CLI + Python SDK).

---

## Key Improvements & Features

### 1. 🚀 Native Persistent Single-Process Driver (`AgyBackend`)
- **Zero Cold Start**: Uses `agy --input-format stream-json --output-format stream-json` with native stdin event dispatch (`{"event":"user","message":{"content":"..."}}`).
- The `agy` subprocess is spawned **once** during `startSession` and kept alive throughout multi-turn conversations (PID remains unchanged).
- Multi-turn turn latency drops from **4~6s** (due to keyring re-reading and eligibility checks) to **~1.0s**.
- Automatic recovery: if the subprocess exits unexpectedly between turns, it is automatically respawned and seamlessly resumes via `--conversation <id>`.

### 2. ⚡ Full Stream-JSON Protocol & Deterministic Context Binding
- Replaced fragile filesystem scanning (`last_conversations.json`) with deterministic extraction of `conversation_id` directly from `agy`'s first `init` event.
- Binds `agyConversationId` to Happy's Session metadata and ACP envelopes.
- Full streaming support for:
  - Text token deltas (`model-output`)
  - Extended thinking / reasoning logs (`event: thinking`)
  - Tool calls & tool execution results (`tool-call`, `tool-result`)
  - Token counts and usage stats (`token_count`, `result`)

### 3. 🛡️ Auto-Retry on Transient Startup / Eligibility EOF Errors
- Built-in exponential backoff retry for transient network errors during startup (e.g., `Eligibility check failed: ... EOF`, TLS timeouts, rate limits).
- Completely eliminates transient startup error banners on mobile/web UI.

### 4. 🏷️ Automatic Session Title & Summary Generation
- Extracts clean summaries from the initial user prompt and persists them to session metadata (`metadata.summary`), ensuring named sessions in the conversation drawer.

### 5. 🔌 Dual-Engine Architecture (`createAgyBackend` & `AgySdkBackend`)
- **CLI Engine (Default / Google OAuth Subscription)**: Zero configuration required for Google subscription accounts via `agy` CLI binary.
- **SDK Engine (`google-antigravity` Python SDK)**: Persistent WebSocket bridge (`bridge/server.py`) activated automatically when `GEMINI_API_KEY` is provided or `HAPPY_AGY_ENGINE=sdk`.

---

## Architecture Overview

```text
┌─────────────────────────────────────────────────────────────┐
│ Happy Session Runner (Persistent)                           │
│                                                             │
│   createAgyBackend Factory                                  │
│     ├── AgyBackend (Persistent Stream-JSON CLI Engine)      │
│     └── AgySdkBackend (Persistent WebSocket Python SDK)     │
└──────────────────────────────┬──────────────────────────────┘
                               │ stdin / stdout NDJSON
                               ▼
┌─────────────────────────────────────────────────────────────┐
│ agy CLI / Python Bridge (Single Process, PID unchanged)     │
│   • Turn 1: Handshake & generate                            │
│   • Turn 2: Stdin dispatch -> 1s instantaneous streaming   │
│   • Turn N: Full context preserved in memory                │
└─────────────────────────────────────────────────────────────┘

──────

Testing & Verification

[✓] Unit Tests:
• src/agy/AgyBackend.test.ts (persistent multi-turn lifecycle & auto-retry)
• src/agy/AgySdkBackend.test.ts (Python bridge streaming & tool calls)
• src/agy/createAgyBackend.test.ts (engine resolution logic)
• src/agy/streamJson.test.ts (NDJSON event parser)
• src/agy/discoverModels.test.ts (model catalog discovery)
• src/agy/title.test.ts (summary extraction)
[✓] Full CLI Suite: All 88 test files (819+ unit tests) passed 100%.
[✓] End-to-End Live Testing:
• Verified multi-turn live conversation with Google subscription account over stdio NDJSON.
• Verified instant token generation on consecutive turns in a single PID.
• Tested daemon startup and remote mode synchronization with Happy Web/Mobile.