feat(v2): OpenCode agent provider

- Add OpenCodeProvider (SSE, session resume, MCP via mcp-to-opencode)
- Register opencode in factory; AGENT_PROVIDER passthrough from DB
- Host: XDG mount, NO_PROXY merge, OPENCODE_* env for opencode sessions
- Dockerfile: opencode-ai CLI; docs checklist + architecture diagram
- Skill add-opencode for v2; AgentProviderName in src/types.ts

Made-with: Cursor

container/agent-runner/package.json

@@ -11,6 +11,7 @@
   "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"
   },