fix(opencode): migrate adapter to the OpenCode v2 plugin API

[MwC-Trexx]

Summary

The OpenCode adapter does not load at all on opencode >= 2.0. This migrates it to the v2 plugin API and to the v2 event taxonomy.

Two independent changes landed in v2 and both are required for the adapter to work.

1. Plugin module format

v2 removed the v1 server plugin API entirely. Local plugins under ~/.config/opencode/plugin{,s}/ are decoded by packages/core/src/plugin/module.ts, which requires the default export to be a definition object:

Schema.Struct({
  default: Schema.Union([
    Schema.Struct({ id: Schema.String, effect: ... }),
    Schema.Struct({ id: Schema.String, setup: ... }),
  ]),
})

This adapter's default export is a bare async function (the v1 shape), so it fails:

PluginModule.LoadError: Plugin must export a default definition with an id and
an effect or setup function.
cause: SchemaError(Expected object at ["default"])

Expected object rather than Missing key is the tell — the key is present, it just is not an object.

2. Event taxonomy

Every event the adapter switched on is gone. session.idle survives only as a // deprecated definition, and session.status is defined but never published by the server. v2 payloads also carry the event body under data rather than properties.

peon-ping hook event	v1 event (removed)	v2 event used instead
SessionStart	session.created	first session.execution.started
UserPromptSubmit	session.status (busy)	session.execution.started
Stop	session.idle	session.execution.succeeded
PostToolUseFailure	session.error	session.execution.failed
PermissionRequest	permission.asked	permission.asked (unchanged)
Notification	question.asked	form.created

Behaviour changes to be aware of

• PostToolUseFailure now fires for any failed execution, not only tool errors — there is no longer a per-tool error event. The task.error peon.sh category is unchanged, so this is a superset of the old trigger.
• session.execution.started doubles as both SessionStart and UserPromptSubmit. SessionStart is emitted first so the existing 3s debounce suppresses the UserPromptSubmit immediately after it, preserving pre-v2 behaviour.
• The elicitation surface was renamed question.* -> form.*, and questions are no longer nested under a questions key. Only the request id is read, and it is still never forwarded to peon.sh (covered by the existing sentinel test).
• The adapter is now untyped. @opencode-ai/plugin is not resolvable from the plugins directory, so depending on it would break loading.

Verification

• bun run test in adapters/opencode: 59 passed (49 internals + 10 adapter).
• Live against opencode v2.0.16: the plugin loads with no failed to load plugin entry, and session.execution.started produces a real SessionStart dispatch to peon.sh ([route] category=session.start suppressed=False).
• All six mapped hook events were driven through a fake v2 host context and observed reaching peon.sh with the correct hook_event_name, cwd and notification_type.

Also added

docs/opencode-v2-events.md documenting the schema, the payload shape change, and the full mapping table.

Note for the installer

adapters/opencode.sh downloads this file from main, so it will pick up this change once merged. No installer change is needed.

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