[FEAT] Wave 2: Plugin integration (build.ts, EmbeddingService sidecar mode, non-blocking embed, TUI)

[four-bytes-robby]

Part of EPIC #117

Tasks:

• W2.1 build.ts integration (download embed-sidecar from GitHub Releases)
• W2.2 EmbeddingService sidecar mode (HTTP client, /status, /embed, /shutdown)
• W2.3 Non-blocking embed flow (hash result immediately, background sidecar init)
• W2.4 TUI status integration (show sidecar phase/progress in status bar)
• W2.5 Configuration (opencode.json + BRAIN_EMBED_SIDECAR_PORT, idle timeout)