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>

AGENTS.md

@@ -21,6 +21,7 @@ The Awesome GitHub Copilot repository is a community-driven collection of custom
 ├── hooks/            # Automated workflow hooks (folders with README.md + hooks.json)
 ├── workflows/        # Agentic Workflows (.md files for GitHub Actions automation)
 ├── plugins/          # Installable plugin packages (folders with plugin.json)
+├── extensions/       # Canvas extensions (each with extension.mjs and plugin metadata)
 ├── docs/             # Documentation for different resource types
 ├── eng/              # Build and automation scripts
 └── scripts/          # Utility scripts
@@ -81,6 +82,19 @@ All agent files (`*.agent.md`) and instruction files (`*.instructions.md`) must
 - Asset files should be reasonably sized (under 5MB per file)
 - Skills follow the [Agent Skills specification](https://agentskills.io/specification)
 
+#### Canvas Extensions (extensions/\*)
+
+- Each extension folder must include `extension.mjs`
+- Extension metadata must live at `.github/plugin/plugin.json`
+- Extension `plugin.json` **must** follow the convention:
+  - `name`, `description`, `version` are required
+  - `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))
+  - Optional: `author`, `keywords` fields
+  - **Must not** include `x-awesome-copilot` field (use convention-based `assets/preview.png` only)
+- Each extension must have `assets/preview.png` as the primary visual asset
+- Do not add `canvas.json`; website metadata is sourced from `.github/plugin/plugin.json`
+
 #### Hook Folders (hooks/\*/README.md)
 
 - Each hook is a folder containing a `README.md` file with frontmatter
@@ -160,6 +174,14 @@ When adding a new agent, instruction, skill, hook, workflow, or plugin:
 5. Run `npm run build` to update README.md and marketplace.json
 6. Verify the plugin appears in `.github/plugin/marketplace.json`
 
+**For Canvas Extensions:**
+
+1. Create/update the extension in `extensions/<extension-id>/` with `extension.mjs`
+2. Add `.github/plugin/plugin.json` metadata (required: `name`, `description`, `version`, `logo: "assets/preview.png"`, `extensions: "."`; optional: `author`, `keywords`)
+3. Ensure `assets/preview.png` exists as the primary visual asset
+4. Run `npm run plugin:validate` to validate plugin and extension metadata
+5. Run `npm run build` to regenerate website data and marketplace output
+
 **For External Plugins:**
 
 1. Do not open a direct PR that edits `plugins/external.json` for a public third-party plugin submission