[FEATURE]: ACP can call a specified opencode server

[georgeharker]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

This allows use in multi-editor contexts like neovim where the app is just a connection to a single opencode instance - which can be done for cli opencode, but currently not acp mode.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Better support remote/server-client use-cases from the CLI #8890: [FEATURE]: Better support remote/server-client use-cases from the CLI - Directly addresses improving server-client workflows from the command line
• [FEATURE]: Attach to authenticated OC Server #8458: [FEATURE]: Attach to authenticated OC Server - Foundational work for multi-client server connection scenarios
• [FEATURE]: Server Registry & Auto-Discovery for Persistent Sessions #8948: [FEATURE]: Server Registry & Auto-Discovery for Persistent Sessions - Requests persistent session management for multiple client connections
• [FEATURE]: Named Remote Connections with Session Origin Tracking #6461: [FEATURE]: Named Remote Connections with Session Origin Tracking - Requests infrastructure for multi-client remote connection scenarios
• [FEATURE]: Support subagent information in ACP #11576: [FEATURE]: Support subagent information in ACP - Related to enabling ACP-based multi-editor contexts

Feel free to ignore if none of these address your specific case.