fix: install pnpm via official install script (#12)

* feat: add opencode and tmux to Docker container

- Install tmux via apt for interactive coding agent sessions
- Install opencode-ai globally via npm for delegating coding tasks
- Configure opencode at startup: maps OPENCODE_GO_API_KEY env var
  to opencode's google provider via .opencode.json
- tmux enables multi-turn interactive opencode sessions
  (one-shot tasks via `opencode run`, interactive via tmux)

* fix: remove wrong GEMINI_API_KEY mapping from entrypoint

OpenCode Go is OpenCode's own subscription service, not Google Gemini.
The OPENCODE_GO_API_KEY is already set as a K8s secret and opencode
natively recognizes the opencode-go provider. No extra config needed.

Models use format: opencode-go/<model-id> (e.g. opencode-go/qwen3.6-plus)

* remove unnecessary OpenCode config comments from entrypoint.sh

Author: bot-ted

Dockerfile

@@ -55,7 +55,8 @@ RUN npm install --prefer-offline --no-audit && \
     (cd web && npm install --prefer-offline --no-audit) && \
     (cd ui-tui && npm install --prefer-offline --no-audit) && \
     npm cache clean --force && \
-    npm install -g opencode-ai@latest pnpm
+    npm install -g opencode-ai@latest && \
+    curl -fsSL https://get.pnpm.io/install.sh | sh -
 
 # ---------- Source code ----------
 # .dockerignore excludes node_modules, so the installs above survive.