Add OpenCode agent tracking

[scross01]

OpenCode sessions now show Running, Needs input, and Idle in the Séance sidebar, with permission/question prompts and completion notifications routed to the correct pane. Tracking handles resumed sessions, subagents, concurrent root sessions, retries, errors, and process exit.

The wrapper injects the bundled plugin for each launch while preserving existing OpenCode configuration, plugins, credentials, and history. Plugin calls are ordered and time-limited so unavailable Séance integration cannot stall OpenCode. A settings toggle controls integration for new panes; remote attachments and --pure runs pass through without tracking.

Validation:

• Build and 170 Zig unit tests passed; 24 E2E tests passed under Xvfb.
• Python wrapper, native socket, installed OpenCode, and GUI suites passed across local runs; only the optional live Codex smoke check was skipped. The plugin suite includes 12 Node tests.
• OpenCode 1.18.26 with OpenRouter GLM-5.3-flash verified real permission/question prompts, resumed sessions, subagents, independent panes, completion, and exit cleanup. The maintainer also verified it manually.
• An upstream OpenCode/Bun crash at a 19-column terminal width was reproduced with plain opencode --pure; normal-width panes passed, and tracking clears after the crashed process exits.

Closes #9

[scross01]

@no1msd I have additional working updates that add support for KiloCode, and MimoCode which are based on OpenCode, and well as agent support for Mistral Vibe Code if your looking to adding more agents to Seance.

[no1msd]

Thanks @scross01 for contributing OpenCode support. Landed as 8e28286 with your authorship preserved.

The final implementation uses per-launch plugin injection and covers configuration preservation, permission/question prompts, resumed sessions, subagents, concurrent sessions, and exit cleanup. CI passed on the final PR revision, alongside local tests and real OpenCode 1.18.26 / OpenRouter GLM-5.3-flash testing; I also verified it manually.

Fixed on main.

[scross01]

@no1msd I played around with adding several more agents using different patterns to encapsulate the agent state and activity, including adding some basic subagent support for OpenCode and others - it diverged a bit from this original PR.
https://github.com/scross01/seance-fork

I've also been experimenting with adding an embedded browser. I'd be happy to help work some functionality back in to the main seance repo.