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/Dockerfile

@@ -18,7 +18,6 @@ ARG INSTALL_CJK_FONTS=false
 ARG CLAUDE_CODE_VERSION=2.1.112
 ARG AGENT_BROWSER_VERSION=latest
 ARG VERCEL_VERSION=latest
-ARG OPENCODE_VERSION=latest
 ARG BUN_VERSION=1.3.12
 
 # ---- System dependencies -----------------------------------------------------
@@ -80,8 +79,7 @@ RUN --mount=type=cache,target=/root/.cache/pnpm \
     pnpm install -g \
         "@anthropic-ai/claude-code@${CLAUDE_CODE_VERSION}" \
         "agent-browser@${AGENT_BROWSER_VERSION}" \
-        "vercel@${VERCEL_VERSION}" \
-        "opencode-ai@${OPENCODE_VERSION}"
+        "vercel@${VERCEL_VERSION}"
 
 # ---- agent-runner ------------------------------------------------------------
 WORKDIR /app