feat(mcp): allow agent to self-modify its own MCP config

[justrach]

Summary

The agent should be able to add, remove, and toggle MCP servers in its own config during a session — without the user having to manually edit JSON or exit the TUI.

Use cases

• Agent discovers it needs a tool (e.g., database MCP) and can add it on the fly
• Agent disables a noisy/slow MCP server that's not relevant to the current task
• Per-agent tool scoping: an agent enables only the MCP servers it needs

How opencode does it

Agents can enable/disable specific MCP tools within their agent config section. A server can be disabled globally but selectively enabled for a particular agent.

Proposed approach

• Expose MCP config modification as a built-in tool the agent can call
• Operations: add_server, remove_server, enable_server, disable_server, list_servers
• Changes persist to the project's MCP config file
• Security: require user confirmation before adding new servers (arbitrary command execution risk)