feat(examples): add ArchViewer example plugin for D2 diagram generation

examples/plugins/archviewer/README.md

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+# 👷‍♂️ Antigravity ArchViewer Plugin
+
+**ArchViewer** is a zero-token, privacy-first architectural diagram generator for [Google Antigravity](https://github.com/google-antigravity/antigravity-cli).
+
+It intelligently watches your agentic brainstorming sessions and, the moment an architecture plan is solidified, autonomously drafts and renders a professional, full-color system topology map using [D2 (Declarative Diagramming)](https://d2lang.com/)—without spending a single LLM token on image generation.
+
+## 🌟 Value Proposition for the Developer Community
+
+1. **Zero-Token High-Fidelity Rendering**: Spare yourself from wasting expensive image-generation tokens or dealing with messy text generation. The agent drafts a deterministic D2 file, and local rendering takes over.
+2. **Absolute Data Privacy**: Sensitive cloud topologies and node relationships never leave your machine to be rendered by third-party web services.
+3. **Diagrams-as-Code (GitOps Ready)**: The intermediate `architecture.d2` means the layout is natively version-controllable. Manually tweak the file, PR the changes, and generate diffs in your CI/CD pipelines.
+4. **Frictionless Portability**: Works out of the box locally provided the `d2` compiler is installed.
+
+## 📦 Installation
+
+Ensure you have D2 installed on your system (e.g., `brew install d2` on macOS).
+
+Copy this directory structure into your workspace's `.agents` or your global `~/.gemini/config` directory:
+
+```text
+.agents/plugins/archviewer/
+├── plugin.json
+├── README.md
+├── architecture.d2       (Auto-generated)
+├── logo.svg              (Official Chat Logo)
+├── rules/
+│   └── AGENTS.md           (Agent instructions)
+└── skills/
+    └── arch/
+        └── SKILL.md        (Slash command definition)
+```
+
+## 🚀 How it Works
+
+When chatting with Antigravity:
+1. Brainstorm your architecture.
+2. When the plan is **solidified** (e.g., you say "Looks good, let's build it"), Antigravity automatically outputs an `architecture.d2` file.
+3. Antigravity runs the D2 compiler to generate an `architecture.png` file and automatically opens it.
+4. Type `/arch` in the chat at any time to instantly pop open the latest generated diagram on your OS.

examples/plugins/archviewer/plugin.json

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+{
+  "name": "archviewer",
+  "description": "D2 declarative architecture diagram builder.",
+  "author": "gretchenboria"
+}

examples/plugins/archviewer/skills/arch/SKILL.md

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+---
+name: arch
+description: Generates a D2 architectural diagram based on the current conversation and opens it.
+---
+
+# ArchViewer Generation Skill
+
+When the `/arch` slash command is used or when this skill is invoked:
+1. Analyze the current conversation and proposed system architecture.
+2. Use the `write_to_file` tool to draft a declarative D2 diagram in `architecture.d2` within the current workspace or artifacts directory.
+   - **Important Layout:** Strictly use `direction: down` (vertical) without any exceptions.
+   - **Important Styling (Light Background + Max Contrast):** Do not use default colors. Use a modern, light-themed aesthetic with high-contrast accents. Exclusively use this precise color palette:
+     - Primary accents (Nodes/Shapes): `#ED1B76` (Vivid Pink) and `#037A76` (Deep Teal)
+     - Backgrounds/Containers: `#F4F4F4` (Light Grey) or `#FFFFFF` (White)
+     - Strokes/Borders: Must be `#1F1F1F` (Dark Grey/Black) with `stroke-width: 3` for sharp visibility against the light background.
+     - Text: MUST explicitly set `font-color: "#FFFFFF"` (Stark White) on the colored nodes, but use `#1F1F1F` (Dark Grey/Black) for text on the light backgrounds or containers.
+   - Use shapes like `circle`, `hexagon`, `cylinder`, and `square` where symbolically appropriate. Do not use unsupported shapes (like `triangle` for node shapes).
+3. Use the `run_command` tool to compile and open the diagram using the exact path written in step 2:
+   `if command -v d2 >/dev/null 2>&1; then d2 --pad 50 "architecture.d2" "architecture.png" && { command -v open >/dev/null && open "architecture.png" || xdg-open "architecture.png"; }; else echo "D2_MISSING"; fi`
+4. If the command output includes `D2_MISSING`, inform the user that the D2 compiler is required and provide the installation command: `curl -fsSL https://d2lang.com/install.sh | sh -s --` or `brew install d2`. Do NOT proceed to the next step.
+5. Briefly summarize the architecture that was generated.