feat(mcp): Phase 2 — graph mutation tools

[staging-devin-ai-integration]

Summary

Follow-up from #358 (Phase 1 landed in #359).

Phase 2 adds mutation tools to the MCP server, enabling AI agents to modify running pipelines:

Proposed tools

• add_node — add a node to a running session
• remove_node — remove a node from a running session
• connect_nodes — create a connection between nodes
• disconnect_nodes — remove a connection
• update_node_params — modify node parameters at runtime
• update_pipeline — apply a full YAML diff to a running session

Design considerations

• Must reuse existing EngineControlMessage flow
• Need to handle graph consistency (reject invalid mutations)
• Should broadcast WebSocket events for each mutation
• Permission checks (modify_sessions) apply

Dependencies

• Phase 1 MCP infrastructure (feat: implement MCP server #359)