feature: connect to provider's `opencode` when it starts

[nickjvandyke]

Did you check the docs?

• I have read all the docs

Is your feature request related to a problem? Please describe.

Split from #118

Mostly addressed by the new server selection feature. Prior to that, users were at the mercy of auto-selection.

However, it's slightly inconvenient to start the provider and then have to manually select its server (via require("opencode").select_server()).

Note this only surfaces when other opencodes are already running. If the provider is the only opencode, the plugin will default to it.

Describe the solution you'd like

When the provider starts, the plugin should connect to its opencode server. Pretty safe to assume that's the one the user wants to interact with.

Describe alternatives you've considered

Manually select the server, or start the provider before other opencodes.

Additional context

Relevant PRs:

• fix(tmux): use pane PID to find correct opencode server port #119
• feat(tmux): add fallback server discovery for split panes #150