feat: migrate to OpenCode v2 plugin API

[ysm-dev]

Summary

• Migrate the plugin to @opencode/plugin with a stable Plugin.define() entrypoint and public v2 session/tool hooks.
• Use ctx.generate.text() for translation so OpenCode owns provider selection, model variants, SQLite credentials, and OAuth refresh/persistence; remove the v1 auth-file readers and custom provider/OAuth adapters.
• Preserve inline bilingual assistant replies through public HTTP response hooks for OpenAI Responses (including Codex), Chat Completions, Anthropic Messages, and Gemini. Both clients consume the normal persisted transcript.
• Restore English-only model context using recorded translations, retain question-form translation, and persist activation/history records across restarts and forks.
• Bump the plugin to 2.0.0, update configuration and packaging, and add native-parser tests plus an isolated real-host smoke test to CI.

Compatibility and behavior changes

• Verified against OpenCode 2.0.3. The installed older opencode2 v0.0.0-dev-18322 did not pass the host smoke test.
• Configure plugins using { package, options } entries. Local builds must point to the dist directory, not dist/index.js.
• HTTP hooks disable the session WebSocket fast path for the location, including inactive translation sessions.
• Unknown endpoints, non-SSE responses, and binary protocols such as Bedrock Converse retain English output with a server diagnostic.
• Start a fresh v2 session when upgrading; v1 activation metadata and historical bilingual trailers are not migrated. OpenCode v1 users should remain on opencode-translate@1.

Verification

• bun run check:ci
• bun run typecheck
• bun run knip
• bun run test:coverage — 76 passing tests, 100% function coverage and 99.16% line coverage
• bun run build
• bun run test:package — install the packed artifact and exercise the v2 entrypoint
• bun run test:host with OpenCode 2.0.3 — actual plugin activation, SQLite credential storage/rotation, bilingual persisted messages, English-only model requests, and restart recovery
• git diff --check

The host test uses an isolated server/database and a local fake provider. Native stream tests use OpenCode’s actual protocol parsers. Browser and terminal visual rendering were not separately automated.

Reference: https://opencode.ai/v2/docs/build/plugins/migrate-v1