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/package.json

@@ -11,7 +11,6 @@
   "dependencies": {
     "@anthropic-ai/claude-agent-sdk": "^0.2.92",
     "@modelcontextprotocol/sdk": "^1.12.1",
-    "@opencode-ai/sdk": "^1.4.3",
     "cron-parser": "^5.0.0",
     "zod": "^4.0.0"
   },