acp: implement session lifecycle (initialize, new, load, prompt)

[bug-ops]

Parent: #762

Task

Implement the core ACP session lifecycle: capability negotiation, session creation/loading, and prompt handling.

Acceptance criteria

• server.rs — AcpServer struct: dispatches incoming JSON-RPC methods to handlers
• session.rs — AcpSession struct: holds session state, message history, config options
• initialize handler: exchange AgentCapabilities / ClientCapabilities, report supported features
• session/new handler: create session, return session ID
• session/load handler: restore session from persisted state (SQLite via zeph-memory)
• session/prompt handler: accept user prompt, bridge to agent loop via LoopbackChannel
• session/cancel notification: forward cancellation to CancellationToken
• session/set_mode handler: map to agent modes if applicable
• Integration with LoopbackChannel::pair() pattern (same as A2A TaskProcessor)

[bug-ops]

Implemented in #774: ZephAcpAgent with initialize, new_session, prompt, cancel, load_session