Opencode V2 plugin support

[bbartels]

Short description

Opencode v2 plugin support

What problem does this solve?

Opencode v2 has a fundamentally different plugin api from v1. I've been using the opencode v2 beta for a while, but am missing magic-context support.

Proposed solution

Add a magic-context opencodev2 plugin

Alternatives considered

No response

Area

OpenCode plugin

Additional context

No response

[alfonso-magic-context]

Adding a concrete item to the v2 wishlist from #268: per-model (or per-request) tool description resolution. In the v1 surface, tool registration is once-per-process, so v0.35.0's prompt-surface presets can route the guidance block per model but not tool text. We plan the v2 implementation once the SDK stabilizes.

[coleleavitt]

Verified against current published OpenCode packages: there is not yet a supported external V2 plugin ABI that exposes the lifecycle surfaces Magic Context requires. @opencode-ai/plugin@1.18.18 exports ./v2/promise and ./v2/effect, but their PluginContext only provides registration domains and lacks client/directory access, tools, events, config, message transforms, compaction, and tool lifecycle hooks. @opencode-ai/sdk/v2 is an HTTP SDK, not a plugin ABI. The official external loader still discovers the V1 contract, and current docs still document V1.

A speculative ./v2 adapter would not be loadable or feature-complete, so no safe code change exists yet. Magic Context should implement this when OpenCode publishes and documents an external V2 loader contract with message-transform and tool/event lifecycle access. Current matrix: V1 >=1.15.0 supported; V2 SDK usable internally; V2 plugin API blocked upstream.