refactor: migrate plugins to Claude Code spec format

- Move plugin manifests from .github/plugin/ to .claude-plugin/
- Convert items[] to Claude Code spec fields (agents, commands, skills)
- Rename tags to keywords, drop display/featured/instructions from plugins
- Delete all symlinks and materialized files from plugin directories
- Add eng/materialize-plugins.mjs to copy source files into plugin dirs at publish time
- Add .github/workflows/publish.yml for staged->main publishing
- Update CI triggers to target staged branch
- Update validation, creation, marketplace, and README generation scripts
- Update CONTRIBUTING.md and AGENTS.md documentation
- Include all new content from main (polyglot-test-agent, gem-browser-tester,
  fabric-lakehouse, fluentui-blazor, quasi-coder, transloadit-media-processing,
  make-repo-contribution hardening, website logo/gradient changes)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

CONTRIBUTING.md

@@ -2,31 +2,6 @@
 
 Thank you for your interest in contributing to the Awesome GitHub Copilot repository! We welcome contributions from the community to help expand our collection of custom instructions and prompts.
 
-## Prerequisites
-
-### Windows Users: Enable Symlinks
-
-This repository uses symbolic links for plugins. On Windows, you need to enable symlink support before cloning:
-
-1. **Enable Developer Mode** (recommended):
-   - Open **Settings** → **Update & Security** → **For developers**
-   - Enable **Developer Mode**
-   - This allows creating symlinks without administrator privileges
-
-2. **Configure Git to use symlinks**:
-   ```bash
-   git config --global core.symlinks true
-   ```
-
-3. **Clone the repository** (after enabling the above):
-   ```bash
-   git clone https://github.com/github/awesome-copilot.git
-   ```
-
-> **Note:** If you cloned the repository before enabling symlinks, the symlinks will appear as plain text files containing the target path. You'll need to delete the local repository and re-clone after enabling symlink support.
-
-**Alternative for older Windows versions:** If Developer Mode is not available, you can run Git Bash as Administrator, or grant your user the "Create symbolic links" privilege via Local Security Policy (`secpol.msc` → Local Policies → User Rights Assignment → Create symbolic links).
-
 ## How to Contribute
 
 ### Adding Instructions
@@ -138,11 +113,11 @@ Skills are self-contained folders in the `skills/` directory that include a `SKI
 
 ### Adding Plugins
 
-Plugins group related prompts, agents, and skills around specific themes or workflows, making it easy for users to install comprehensive toolkits via GitHub Copilot CLI.
+Plugins group related agents, commands (prompts), and skills around specific themes or workflows, making it easy for users to install comprehensive toolkits via GitHub Copilot CLI.
 
 1. **Create your plugin**: Run `npm run plugin:create` to scaffold a new plugin
 2. **Follow the naming convention**: Use descriptive, lowercase folder names with hyphens (e.g., `python-web-development`)
-3. **Add your content**: Add agents, commands (prompts), and skills to the plugin folder using symlinks to existing repo files
+3. **Define your content**: List agents, commands, and skills in `plugin.json` using the Claude Code spec fields
 4. **Test your plugin**: Run `npm run plugin:validate` to verify your plugin structure
 
 #### Creating a plugin
@@ -155,41 +130,37 @@ npm run plugin:create -- --name my-plugin-id
 
 ```
 plugins/my-plugin-id/
-├── .github/plugin/plugin.json  # Plugin metadata
-├── README.md                   # Plugin documentation
-├── commands/                   # Symlinked prompt files
-├── agents/                     # Symlinked agent files
-└── skills/                     # Symlinked skill folders
+├── .github/plugin/plugin.json  # Plugin metadata (Claude Code spec format)
+└── README.md                   # Plugin documentation
+```
+
+> **Note:** Plugin content is defined declaratively in plugin.json using Claude Code spec fields (`agents`, `commands`, `skills`). Source files live in top-level directories and are materialized into plugins by CI.
+
+#### plugin.json example
+
+```json
+{
+  "name": "my-plugin-id",
+  "description": "Plugin description",
+  "version": "1.0.0",
+  "keywords": [],
+  "author": { "name": "Awesome Copilot Community" },
+  "repository": "https://github.com/github/awesome-copilot",
+  "license": "MIT",
+  "agents": ["./agents/my-agent.md"],
+  "commands": ["./commands/my-command.md"],
+  "skills": ["./skills/my-skill/"]
+}
 ```
 
 #### Plugin Guidelines
 
-- **Use symlinks**: Plugin content should be symlinks to source files in agents/, prompts/, skills/ directories
-- **Valid references**: All items referenced in plugin.json must exist in the repository
+- **Declarative content**: Plugin content is specified via `agents`, `commands`, and `skills` arrays in plugin.json — source files live in top-level directories and are materialized into plugins by CI
+- **Valid references**: All paths referenced in plugin.json must point to existing source files in the repository
+- **Instructions excluded**: Instructions are standalone resources and are not part of plugins
 - **Clear purpose**: The plugin should solve a specific problem or workflow
 - **Validate before submitting**: Run `npm run plugin:validate` to ensure your plugin is valid
 
-### Working with Plugins
-
-Plugins are installable packages that contain symlinked agents, commands (prompts), and skills organized around a specific theme or workflow.
-
-#### Plugin Structure
-
-```plaintext
-plugins/<plugin-name>/
-├── .github/plugin/plugin.json  # Plugin metadata
-├── README.md                   # Plugin documentation
-├── agents/                     # Symlinks to agent files (.md)
-├── commands/                   # Symlinks to prompt files (.md)
-└── skills/                     # Symlinks to skill folders
-```
-
-#### Plugin Guidelines
-
-- **Symlinks, not copies**: Plugin files are symlinks to the source files, avoiding duplication
-- **Instructions excluded**: Instructions are not currently supported in plugins
-- **Validate before submitting**: Run `npm run plugin:validate` to ensure your plugin is valid
-
 ## Submitting Your Contribution
 
 1. **Fork this repository**
@@ -198,11 +169,14 @@ plugins/<plugin-name>/
 4. **Run the update script**: `npm start` to update the README with your new file (make sure you run `npm install` first if you haven't already)
    - A GitHub Actions workflow will verify that this step was performed correctly
    - If the README.md would be modified by running the script, the PR check will fail with a comment showing the required changes
-5. **Submit a pull request** with:
+5. **Submit a pull request** targeting the `staged` branch with:
    - A clear title describing your contribution
    - A brief description of what your instruction/prompt does
    - Any relevant context or usage notes
 
+> [!IMPORTANT]
+> All pull requests should target the **`staged`** branch, not `main`.
+
 > [!NOTE] 
 > We use [all-contributors](https://github.com/all-contributors/all-contributors) to recognize all types of contributions to the project. Jump to [Contributors Recognition](#contributor-recognition) to learn more!