feat(plugin-runtime): add plugin isolation IPC infrastructure
- Protocol layer: Envelope model with Pydantic schema, MsgPack/JSON codecs, unified error codes
- Transport layer: cross-platform IPC abstraction with 4-byte length-prefixed framing (UDS + TCP fallback)
- Host: RPC server, policy engine, circuit breaker, capability service, supervisor with hot-reload
- Runner: RPC client, plugin loader, process entry point
- Tests: 16 passing tests covering protocol, transport, host, and E2E handshake
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