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

docs/v2-checklist.md

@@ -34,7 +34,7 @@ Status: [x] done, [~] partial, [ ] not started
 - [x] Mock provider (testing)
 - [x] Provider factory
 - [ ] Codex provider
-- [~] OpenCode provider
+- [x] OpenCode provider
 
 ## Channel Adapters