Epic: HTTP+SSE and WebSocket Transport

[bug-ops]

Summary

stdio transport limits ACP to subprocess mode. HTTP+SSE and WebSocket transports enable standalone server mode, multi-client connections, and remote IDE access.

Critical Path

[Epics 1+2 complete] ──→ [3.1 HTTP+SSE] ──→ [3.2 WebSocket]
                     ──→ [3.3 Transport abstraction] (parallel with 3.1)

• Depends on: Epics 1+2 (Epic: Wire AcpContext into Agent Loop #777, Epic: Session Lifecycle Management #778)
• Parallel with: Epics 4, 5
• 3.1 and 3.3 can run in parallel; 3.2 depends on 3.3

Scope

• Axum HTTP server at /acp with SSE streaming
• WebSocket upgrade on same endpoint
• Acp-Session-Id header routing
• Unified transport trait for stdio/HTTP/WS

Acceptance Criteria

• HTTP+SSE transport functional with IDE client
• WebSocket transport functional
• Common transport abstraction shared by all three modes
• SSE keepalive pings
• Transport session lifecycle (create/destroy)