Plan OpenCode V2 plugin API support

[DanRioDev]

Context

OpenCode V2 is currently beta and intentionally does not load V1 plugins. The migration guide describes the new plugin shape (Plugin.define({ id, setup })) and runtime registration APIs: https://opencode.ai/v2/docs/migrate-v1

opencode-pty currently targets the V1 contract in src/plugin.ts, exporting an async plugin function with PluginContext / PluginResult and depending on @opencode-ai/plugin ^1.3.13.

This issue is a planning request, not an expectation that unstable V2 support ship immediately.

Suggested port plan

• Keep the current V1 entry point while V2 remains beta; add a separate V2 entry point/export.
• Replace the V1 async plugin export with a unique V2 id and setup.
• Register PTY tools through the V2 tool transform/registration surface, preserving existing schemas and descriptions.
• Map command handling and session notifications to V2 command/session APIs.
• Review process ownership and cleanup against V2 session interruption, service restart, and standalone/managed modes.
• Avoid sharing host-specific client calls between V1 and V2; keep the PTY/process core shared and adapters thin.

Compatibility checks

• Plugin appears in opencode2 api get /api/plugin.
• PTY creation, input, resize, output polling, and termination work in opencode2 --standalone.
• Managed service mode behaves the same as standalone mode.
• Session interruption and service shutdown leave no orphan process.
• Invalid or expired PTY IDs return bounded errors rather than crashing the host.
• Existing V1 tests and package entry point remain unchanged.

Would V2 support be in scope once the API settles? I can help test a branch against current opencode2 previews.