fix(opencode): support the OpenCode 2 CLI and plugin API

[aryasaatvik]

Summary

OpenCode 2 is GA and incompatible with Entire's OpenCode integration in two places, so no transcript was ever captured: the generated plugin used the OpenCode 1 server-hook export (rejected with PluginModule.LoadError), and the CLI ran opencode export, which OpenCode 2 answers with a help page and exit 0. This makes the integration target OpenCode 2 end to end.

Changes

• Plugin — emit an OpenCode 2 plugin definition (id + setup): session hooks replace experimental.chat.system.transform, the event subscription replaces the event map, and session-end moves to plugin unload. The SDK types are imported type-only from the public @opencode/plugin entry, so the loader never resolves project node_modules. Drops OpenCode 1 plugin support.
• Commands — try opencode session export / session import (OpenCode 2), validate the export before accepting it, and fall back to opencode export / import (OpenCode 1). A classified failure from the OpenCode 2 attempt (e.g. "Session not found") is preserved rather than overwritten by the fallback's invalid output.
• Schema — normalize both export shapes (info/parts vs type/content plus a message-level text) to the v1 model for reads, and keep chunking/reassembly byte-preserving so the stored raw transcript stays importable by opencode session import.
• e2e — accept OpenCode 2's opencode v2.0.2 version output, and skip the OpenCode 1 plugin-dependency seeding on major ≥ 2, where the V2 API is provided by the binary.

Flow

turn-end hook
  -> opencode session export <id>   (fallback: opencode export <id>)
  -> validate JSON (rejects exit-0 help; keeps a classified failure)
  -> .entire/tmp/<id>.json          (native, importable)
  -> ParseExportSession             (normalized v1 view for prompts/tokens/files)

Review notes

• The v1 command exits 0 with a help page on OpenCode 2, so the result is validated rather than trusted, and the OpenCode 2 attempt's classified error survives the fallback.
• OpenCode 1 plugin support is intentionally dropped; the CLI keeps the v1 export/import command fallback and the e2e harness degrades to config-only seeding on OpenCode 2.
• Verified against a real opencode session export (messages, prompts, modified files, tokens) and an end-to-end hook run in a scratch repo that wrote a 2 MB .entire/tmp/<id>.json.