feat(core): add runtime MCP controls

[opencode-agent]

Summary

• add or replace Location-scoped MCP servers at runtime from a full local or remote config
• connect and disconnect known MCP servers without persisting runtime state to config
• preserve OAuth integration registration and credential-driven reconnects for dynamically added remote servers
• clear cached tools, prompts, and resources and publish lifecycle events when replacing or disconnecting a server

This provides the core runtime operations used by V1 ACP while intentionally leaving protocol/server endpoints for a separate change.

Tests

• bun test test/mcp.test.ts (packages/core)
• bun typecheck (packages/core)
• scoped oxlint and Effect pattern scan