Migrate extension metadata to plugin.json and enforce conventions (#2177)

* Remove pluginRoots property from marketplace.json

The pluginRoots property is not used by install tooling and was only
informational about the extension/plugin source directories. Removing it
simplifies the marketplace.json structure while maintaining all functionality.

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

* Migrate java-modernization-studio to plugin.json and update validation workflow

- Create .github/plugin/plugin.json for java-modernization-studio extension
- Remove legacy canvas.json from java-modernization-studio
- Update validate-canvas-extensions.yml workflow to check for plugin.json instead of canvas.json
- Update workflow to trigger on .schemas/plugin.schema.json changes (instead of canvas.schema.json)
- Remove schema validation logic that relied on canvas.schema.json
- All 12 extensions now use plugin.json for metadata

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

* Add extensions field to all extension plugin.json files

Per https://github.com/github/copilot-agent-runtime/pull/9929, plugins that ship
extensions need to include an extensions field specifying where the extension code
is located. All 12 extensions now have extensions set to '.' to reference the
current directory where extension.mjs is located.

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

* Enforce convention-based extension metadata and remove x-awesome-copilot

- Remove x-awesome-copilot.screenshots from all extension plugin.json files
- Enforce logo=assets/preview.png convention for all extensions
- Enforce extensions=. per copilot-agent-runtime#9929
- Update validate-plugins.mjs to enforce conventions
- Update validate-canvas-extensions.yml workflow with convention checks
- Update AGENTS.md and CONTRIBUTING.md documentation

All 12 extensions validated successfully.

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

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Use standard plugin validation for extensions

Remove the custom extension schema and schema validation helper, and
validate extension plugin.json files through the existing plugin validator
instead. Update workflows to stop depending on the removed schema.

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

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

CONTRIBUTING.md

@@ -11,6 +11,7 @@ Thank you for your interest in contributing to the Awesome GitHub Copilot reposi
   - [Adding Instructions](#adding-instructions)
   - [Adding Agents](#adding-an-agent)
   - [Adding Skills](#adding-skills)
+  - [Adding Canvas Extensions](#adding-canvas-extensions)
   - [Adding Plugins](#adding-plugins)
   - [Adding Hooks](#adding-hooks)
   - [Adding Agentic Workflows](#adding-agentic-workflows)
@@ -137,6 +138,21 @@ Skills are self-contained folders in the `skills/` directory that include a `SKI
 3. **Add optional assets**: Keep bundled assets reasonably sized (under 5MB each) and reference them from `SKILL.md`
 4. **Validate and update docs**: Run `npm run skill:validate` and then `npm run build` to update the generated README tables
 
+### Adding Canvas Extensions
+
+Canvas extensions live in `extensions/<extension-id>/` and are installable through plugin metadata.
+
+1. **Create/update extension metadata**: Add `.github/plugin/plugin.json` in the extension folder
+2. **Use convention-based metadata**: Follow the extension plugin.json structure:
+   - Required: `name` (matching folder name), `description`, `version`
+   - Optional: `author`, `keywords`
+   - `logo` **must** be exactly `"assets/preview.png"` (enforced convention)
+   - `extensions` **must** be exactly `"."` (per [copilot-agent-runtime#9929](https://github.com/github/copilot-agent-runtime/pull/9929))
+   - **Never** include `x-awesome-copilot` field (use convention-based assets only)
+3. **Screenshot requirements**: Create `assets/preview.png` as your primary visual
+4. **Do not add `canvas.json`**: Extension website metadata is now sourced from `.github/plugin/plugin.json`
+5. **Validate before submitting**: Run `npm run plugin:validate` to check compliance with conventions
+
 ### Adding Plugins
 
 Plugins group related agents, commands, and skills around specific themes or workflows, making it easy for users to install comprehensive toolkits via GitHub Copilot CLI.
@@ -183,6 +199,7 @@ plugins/my-plugin-id/
 
 - **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
+- **Optional extension links**: Curated plugins can reference extensions using `x-awesome-copilot.extensions` with paths like `./extensions/<extension-id>`
 - **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