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>

src/providers/index.ts

@@ -5,4 +5,3 @@
 //
 // Skills add a new provider by appending one import line below.
 
-import './opencode.js';