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

src/providers/index.ts

@@ -4,3 +4,5 @@
 // needs (claude, mock) don't appear here.
 //
 // Skills add a new provider by appending one import line below.
+
+import './opencode.js';