feat: add MCP tracking to sessions (#19)

Add mcps field to Session model to track active MCP servers and populate it from container labels in ContainerManager. Enhance MCP remove command to warn when removing servers used by active sessions.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

cubbi/container.py

@@ -107,12 +107,21 @@ class ContainerManager:
                 elif container.status == "created":
                     status = SessionStatus.CREATING
 
+                # Get MCP list from container labels
+                mcps_str = labels.get("cubbi.mcps", "")
+                mcps = (
+                    [mcp.strip() for mcp in mcps_str.split(",") if mcp.strip()]
+                    if mcps_str
+                    else []
+                )
+
                 session = Session(
                     id=session_id,
                     name=labels.get("cubbi.session.name", f"cubbi-{session_id}"),
                     image=labels.get("cubbi.image", "unknown"),
                     status=status,
                     container_id=container_id,
+                    mcps=mcps,
                 )
 
                 # Get port mappings