Epic: Model Switching and Extension Management

[bug-ops]

Summary

No per-session model switching or runtime extension management. IDE cannot change the LLM model or add/remove MCP servers without restarting the session.

Critical Path

[Epics 1+2 complete] ──→ [5.1 Model switching]      ─┐ (parallel)
                     ──→ [5.2 Extension management]  ─┘

• Depends on: Epics 1+2 (Epic: Wire AcpContext into Agent Loop #777, Epic: Session Lifecycle Management #778)
• Parallel with: Epics 3, 4
• 5.1 and 5.2 are independent

Acceptance Criteria

• session/set_model swaps provider dynamically
• Available models returned in new_session response
• MCP servers added/removed at runtime via custom methods