feat: Gateway RPC methods for twin messaging

[kody-w]

Parent: #22

Methods

• `twin.send` — encrypt + store + broadcast to WebSocket subscribers + queue iMessage sync
• `twin.history` — return encrypted message history for a conversation
• `twin.key` — create/exchange/rotate conversation keys (egg.json format)
• `twin.sync` — bidirectional iMessage ↔ twin store sync
• `twin.export` — portable encrypted conversation export
• `twin.conversations` — list all conversations with metadata

Events (WebSocket push)

• `twin.message` — new message in a conversation
• `twin.status` — delivery status update (syncing → delivered)
• `twin.participant` — participant joined/left

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