refactor(v2): move opencode provider off v2 trunk

v2 ships with only claude baked in. opencode now lives on the `providers`
branch and gets copied in via the /add-opencode skill.

Removed:
- src/providers/opencode.ts
- container/agent-runner/src/providers/{opencode,mcp-to-opencode}.ts + test
- @opencode-ai/sdk from agent-runner package.json + bun.lock
- opencode-ai global install + OPENCODE_VERSION ARG from Dockerfile
- opencode self-registration imports from both provider barrels
- opencode test case from factory.test.ts

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

container/agent-runner/src/providers/factory.test.ts

@@ -3,17 +3,12 @@ import { describe, it, expect } from 'bun:test';
 import { createProvider, type ProviderName } from './factory.js';
 import { ClaudeProvider } from './claude.js';
 import { MockProvider } from './mock.js';
-import { OpenCodeProvider } from './opencode.js';
 
 describe('createProvider', () => {
   it('returns ClaudeProvider for claude', () => {
     expect(createProvider('claude')).toBeInstanceOf(ClaudeProvider);
   });
 
-  it('returns OpenCodeProvider for opencode', () => {
-    expect(createProvider('opencode')).toBeInstanceOf(OpenCodeProvider);
-  });
-
   it('returns MockProvider for mock', () => {
     expect(createProvider('mock')).toBeInstanceOf(MockProvider);
   });