feat(cli): allow disabling the background service - #52107

Summary

• Add opencode service set disabled true to stop the background service and default CLI connections to private standalone servers.
• Route CLI MCP, plugin, and debug commands through the same connection choice; reject pairing while the service is disabled. Explicit --server still works.
• Document the toggle and its reversal.

Verification

• Focused CLI service and connection tests pass, including standalone API/MCP calls, no daemon registration, pairing rejection, and explicit --server override.
• CLI typecheck, lint, website typecheck and generated-doc check pass.
• Full workspace typecheck was blocked by host Node 18 / resource limits in unrelated packages; website build completed compilation but failed during Miniflare teardown on this host.

Requested by: @jlongster (James via Slack)