Support opencode v2 plugin API (dual v1/v2 adapter) #15

Synopsis

opencode 2.0 replaced the plugin API. Thatch's plugin (v1 shape) fails to load on opencode v2 with a PluginModule.LoadError. This issue tracks the migration: dual v1/v2 support in the same @jeffober/thatch package -- no fork, MCP path untouched.

Plan

Full refined plan (CONSENSUS, 4 review rounds):

• Doc: docs/plans/opencode-v2-plugin.md on branch opencode-v2-migration
• Branch: opencode-v2-migration

Why it breaks

• v2 requires a default export { id, setup } (promise API) or { id, effect } (Effect API); the v1 named server export is not read. Verified at tag v2.0.9 (packages/core/src/plugin/module.ts).
• The v2 SDK is a new npm name: @opencode/plugin (@opencode-ai/plugin frozen at 1.18.32).
• Upstream migration doc is explicit: "Port plugins, because V1 plugin implementations do not run in V2."

Approach (from the plan)

1. Merged default export { id, setup, server } -- both loaders consume their own keys; verified against v1.18.32's readV1Plugin and (empirically) v2.0.9's Effect Schema decoder.
2. Lazy dynamic imports so each host's SDK (@opencode-ai/plugin vs @opencode/plugin) only resolves under its own runtime.
3. Thin adapters src/opencode/v1.ts / src/opencode/v2.ts behind a HostCapabilities seam; shared core unchanged.
4. Declared degradation for v2-missing surfaces (child-session delete, session.list/messages, tui commands, toast publish path) with strategies in the plan's capability table.

Acceptance

• Milestone 1: seam extraction, dual entry, shim updates, zero behavior change, mise run check green
• Milestone 2: smoke tests against real opencode-v2 + v1 binaries; open questions resolved
• Milestones 3-5: v2 adapter, toasts-or-degrade, mirrored test suite + Bun 1.4.2 CI/publish legs
• Milestone 6: README "2.0 not yet supported" note removed, dev docs updated, release cut