fix(plugin): migrate plugin/index.ts to Effect layer pattern — imperative init deadlocks after upstream sync #413

Problem

After the 2026-05-07 upstream sync (PR #412), ~/bin/opencode hangs on startup with a black screen when an external plugin is present in ~/.config/opencode/plugin/. Setting OPENCODE_DISABLE_DEFAULT_PLUGINS=1 works around it.

Root Cause

Upstream commit 32a74b72c ("move plugin initialisation to config layer override") introduced Effect-based lazy plugin initialization. Our fork's plugin/index.ts kept the old imperative pattern (await PluginLoader.loadExternal() inside Plugin.init() called synchronously from bootstrap.ts). This creates a deadlock:

Plugin.init() → await Config.get()
  → Config layer construction
    → PluginLoader.loadExternal()
      → await import(plugin)
        → plugin calls client.auth.set()
          → needs HTTP server (not ready yet)
            → DEADLOCK

Fix

Migrate packages/opencode/src/plugin/index.ts to upstream's Effect layer pattern:

1. Replace imperative Plugin.init() with Effect.layer-based Service class
2. Wrap plugin loading in Effect.tryPromise() for proper async handling
3. Use InstanceState.makeState() for per-instance plugin state
4. Remove await Plugin.init() call from bootstrap.ts (Effect layer manages lazily)

Upstream reference: anomalyco/dev:packages/opencode/src/plugin/index.ts

Quality Gates

• Plugin loads without deadlock
• External OAuth plugin (~/.config/opencode/plugin/) works correctly
• OPENCODE_DISABLE_DEFAULT_PLUGINS=1 still works
• Typecheck passes
• Binary starts cleanly

Acceptance Criteria

• ~/bin/opencode starts without black screen hang when external plugin present
• Anthropic OAuth plugin loads and works correctly
• No regression in internal plugin loading