fix: restore external plugin loading in Plugin.init()

[randomm]

What happened

Plugin.init() in packages/opencode/src/plugin/index.ts is a stub that only logs "plugin system stub - init called" and returns. External plugins from ~/.config/opencode/plugins/ are never loaded.

This was introduced in commit 53f683f when we aligned the plugin system with upstream in PR #392. The alignment restored upstream's Effect-based plugin files but left init() as a stub since the fork doesn't use Effect.

Expected behaviour

• Plugin.init() loads internal plugins (CodexAuthPlugin, CopilotAuthPlugin)
• Plugin.init() loads external plugins from config directories via PluginLoader.loadExternal()
• Plugins receive a PluginInput with an SDK client, project info, and Bun shell access
• Plugin load failures are logged but don't crash the system

Root Cause

Commit 53f683f replaced the working init() with a stub:

export async function init() {
  log.info("plugin system stub - init called")
}

All the supporting infrastructure still exists and works:

• Config.get() scans ~/.config/opencode/plugins/*.{ts,js} and populates config.plugin[] (config.ts:460-474, 1338-1345)
• PluginLoader.loadExternal() fully functional (loader.ts:152)
• applyPlugin() defined (index.ts:46-52)
• INTERNAL_PLUGINS defined (index.ts:15)
• PluginInput type from @opencode-ai/plugin (packages/plugin/src/index.ts:26-33)

Implementation

Replace the stub init() in packages/opencode/src/plugin/index.ts:

1. Add imports: Config, Server, Instance, createOpencodeClient
2. Create an SDK client with in-process fetch adapter (no HTTP roundtrip — use Server.App().fetch() directly)
3. Build PluginInput from Instance context
4. Load internal plugins via INTERNAL_PLUGINS array
5. Load external plugins via PluginLoader.loadExternal() with error reporting
6. Update .fork-features/manifest.json with "plugin-loading" feature entry documenting this divergence from upstream

Quality Gate: @ts-ignore

The proposed code uses @ts-ignore for the fetch type incompatibility between Hono and native fetch. Per AGENTS.md, @ts-ignore is forbidden. Use a type-safe approach instead (e.g., a properly typed adapter function or explicit cast with explanatory comment).

Acceptance Criteria

• Plugin.init() loads internal plugins (CodexAuthPlugin, CopilotAuthPlugin)
• Plugin.init() loads external plugins from ~/.config/opencode/plugins/*.{ts,js} via PluginLoader.loadExternal()
• External plugin load failures are logged but don't crash the system
• No @ts-ignore, as any, or other quality gate violations
• bun run typecheck passes with 0 errors
• bun test passes (existing + new tests for plugin loading)
• .fork-features/manifest.json updated with "plugin-loading" feature entry
• bun run .fork-features/verify.ts passes

Definition of Done

• Tests written for plugin loading (TDD preferred)
• 80%+ coverage for new code in init()
• All linting passes
• Fork-features manifest entry created with proper criticalCode markers
• Verified that Plugin.trigger() still works with loaded hooks