Add OpenCode v2 native review panel and model tool

[sidkmenon]

Add a Diffr plugin for OpenCode v2.0.25 using the native session.panel API. Review opens beside the transcript at the host's default half width, with a draggable divider and fullscreen toggle. OpenCode 1.x is unsupported; no sidebar or overlay fallback remains.

The server registers native tool diffr_open. A namespaced plugin RPC event reaches the matching visible terminal, which claims a private local reply socket and acknowledges the loaded comparison or an error. Cancellation, timeout, and session/directory checks prevent unrelated terminals from handling the request.

Selected code appends to the originating unsent draft through the captured public OpenTUI Textarea API. Stable row renderables preserve mouse capture across redraws, so multi-row drags retain both diff sides instead of stopping after the first move. Fullscreen transfer hides the panel and returns focus to chat; reopening preserves review state.

Validation: six behavioral tests pass, including real OpenTUI multi-row selection in unified/split layouts, both drag directions, redraws between events, and preserved-draft/fullscreen behavior. Typecheck passes. Real Ghostty with OpenCode 2.0.25 exercised model open, native split/fullscreen, folds/files/viewed marks, a whole-loop mouse drag, reverse cross-side selection, and Enter into an unsent draft.

Limits: local macOS/Linux transport only; no standalone MCP server; low-level editor capture needs host-version checks. The host forces fullscreen at 80 available columns or fewer. Real Add to chat button and divider dragging remain unverified. The isolated v2 runtime is used because a system Homebrew upgrade requires newer Command Line Tools.

Base: #1024. Next: Herdr #1026.

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Stack: #1023 → #1024 → #1025 → #1026 → #1027 (above #1019).