feat(skills): add nano-banana-pro-openrouter skill

✨ - Generated by Copilot

skills/nano-banana-pro-openrouter/SKILL.md

@@ -0,0 +1,61 @@
+---
+name: nano-banana-pro-openrouter
+description: Generate or edit images via OpenRouter using openai-python with the Gemini 3 Pro Image model. Use for prompt-only image generation, image edits, and multi-image compositing; supports 1K/2K/4K output, saves results to the current working directory, and prints MEDIA lines.
+metadata:
+  emoji: 🍌
+  requires:
+    bins:
+      - uv
+    env:
+      - OPENROUTER_API_KEY
+  primaryEnv: OPENROUTER_API_KEY
+---
+
+# Nano Banana Pro OpenRouter
+
+## Overview
+
+Generate or edit images with OpenRouter using the `google/gemini-3-pro-image-preview` model and the openai-python client. Support prompt-only generation, single-image edits, and multi-image composition. Save results to the current working directory and output MEDIA lines for easy attachment.
+
+### Prompt-only generation
+
+```
+uv run {baseDir}/scripts/generate_image.py \
+  --prompt "A cinematic sunset over snow-capped mountains" \
+  --filename sunset.png
+```
+
+### Edit a single image
+
+```
+uv run {baseDir}/scripts/generate_image.py \
+  --prompt "Replace the sky with a dramatic aurora" \
+  --input-image input.jpg \
+  --filename aurora.png
+```
+
+### Compose multiple images
+
+```
+uv run {baseDir}/scripts/generate_image.py \
+  --prompt "Combine the subjects into a single studio portrait" \
+  --input-image face1.jpg \
+  --input-image face2.jpg \
+  --filename composite.png
+```
+
+## Resolution
+
+- Use `--resolution` with `1K`, `2K`, or `4K`.
+- Default is `1K` if not specified.
+
+## System prompt customization
+
+The skill reads an optional system prompt from `assets/SYSTEM_TEMPLATE`. This allows you to customize the image generation behavior without modifying code.
+
+## Behavior and constraints
+
+- Read the API key from `OPENROUTER_API_KEY` (no CLI flag).
+- Accept up to 3 input images via repeated `--input-image`.
+- Save output in the current working directory. If multiple images are returned, append `-1`, `-2`, etc.
+- Print `MEDIA: <path>` for each saved image. Do not read images back into the response.