[FEATURE]: Allow per-session MCP selection with `serve` and `attach`

[mirsella]

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

When one opencode serve instance has multiple clients connected with opencode attach, the MCP enabled/disabled state is currently shared by clients using the same directory.

Each attached session should be able to independently choose which configured MCP servers are available to it. Disabling an MCP in one session must not change its availability in any other session. The server should continue sharing the underlying MCP processes and connections rather than starting a separate process for every session.

This is distinct from persisting MCP state across restarts and from supplying a different MCP configuration when creating a session.

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• Feature requests must use the feature request template: the title should start with [FEATURE]: and the issue should include the verification checkbox confirming the feature hasn't been suggested before.

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

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[github-actions]

Thanks for updating the issue! It now follows the contributing guidelines and looks good. We appreciate you taking the time to clarify the request.

[github-actions]

Thanks for updating the issue! It now follows the contributing guidelines and looks good to us.