feat: initial pi-review Docker action

Reusable Gitea/GitHub action that runs Pi coding agent for
AI-powered code reviews on pull requests.

- Docker image based on node:24-slim (112 packages)
- Supports built-in providers (zai, anthropic, openai, deepseek, openrouter)
  and custom OpenAI-compatible endpoints
- Generates git diff (excludes lockfiles/generated code by default)
- Posts review as idempotent PR comment (updates existing on re-run)
- Read-only tools only: agent investigates but never modifies code
- 80KB default diff truncation to stay within LLM context windows
- No curl/python3 dependency — uses Node.js for HTTP and JSON

Dockerfile

@@ -0,0 +1,22 @@
+FROM node:24-slim
+
+# node:24-slim has ~88 packages (vs 413 in bookworm).
+# We only add git for diffing. curl and python3 are replaced by Node.js.
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    git \
+    && rm -rf /var/lib/apt/lists/*
+
+# Install Pi globally — baked into the image, no install at runtime
+RUN npm install -g @earendil-works/pi-coding-agent
+
+# Copy action files into the image
+COPY prompts/default.md /action/prompts/default.md
+COPY scripts/review.sh  /action/scripts/review.sh
+COPY entrypoint.sh      /action/entrypoint.sh
+
+RUN chmod +x /action/entrypoint.sh /action/scripts/review.sh
+
+# Disable Pi's startup network calls
+ENV PI_OFFLINE=1
+
+ENTRYPOINT ["/action/entrypoint.sh"]