fix(core): reload changed MCP config

[opencode-agent]

Summary

• subscribe each location-scoped MCP service to config.updated
• compare the previous and next effective MCP server maps before reconciling
• add, replace, or remove only server entries whose effective configuration changed
• preserve API-registered servers that are not owned by config updates
• reconcile once after installing the live subscription so startup-time config changes cannot be lost

Strict reconciliation behavior

Unrelated config edits and semantically unchanged MCP entries do not restart existing MCP connections. Effective server changes—including inherited MCP timeout changes—replace only the affected server. Added and removed config entries affect only their corresponding names.

Verification

• bun test test/mcp.test.ts in packages/core (24 passed)
• bun typecheck in packages/core
• scoped oxlint on the changed implementation and test files (no errors; existing warnings only)

Requested by: @rekram1-node (Aiden Cline via Slack)