skills: bubble plugin skills through the "configure skills" and agent customization view

[connor4312]

Closes #297249

[Copilot]

Pull request overview

This PR adds first-class support for plugin-provided skills/prompt files by introducing a new PromptsStorage.plugin storage type and wiring it through prompt discovery, pickers, and the AI Customization management UI so plugin artifacts show up and are treated as read-only.

Changes:

• Add PromptsStorage.plugin + IPluginPromptPath and plumb plugin prompt files through PromptsService (listing, labeling, telemetry, skill discovery ordering).
• Update plugin discovery to surface skills via skills/<skill>/SKILL.md.
• Update UI surfaces (prompt file pickers + AI Customization management/list) to display plugin items and treat them as read-only (with a plugin icon).

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/vs/workbench/test/browser/componentFixtures/promptFilePickers.fixture.ts	Updates test fixture switch to handle PromptsStorage.plugin.
src/vs/workbench/contrib/chat/common/promptSyntax/service/promptsServiceImpl.ts	Tracks plugin prompt files, exposes plugin storage listing, updates labels/telemetry, and includes plugin skills in discovery ordering.
src/vs/workbench/contrib/chat/common/promptSyntax/service/promptsService.ts	Extends prompt storage and prompt path/source unions to include plugin storage and pluginUri.
src/vs/workbench/contrib/chat/common/promptSyntax/config/promptFileLocations.ts	Adds PromptFileSource.Plugin for classification/telemetry.
src/vs/workbench/contrib/chat/common/plugins/agentPluginServiceImpl.ts	Adjusts plugin skill discovery to expect SKILL.md within skill folders.
src/vs/workbench/contrib/chat/common/chatModes.ts	Serializes/revives custom chat mode sources for plugin-backed agents.
src/vs/workbench/contrib/chat/browser/promptSyntax/pickers/promptFilePickers.ts	Adds plugin section to prompt pickers and adds an exhaustive storage switch.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationManagementEditor.ts	Treats plugin items as read-only in the embedded editor path.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationManagement.contribution.ts	Prevents deleting plugin-provided files (alongside extensions).
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts	Adds plugin grouping/badging so plugin artifacts appear in the management list.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationIcons.ts	Registers a new plugin storage icon.

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationManagement.contribution.ts:204

• This dialog is now shown for both extension and plugin files, but the title/detail strings still say “Extension File” and only mention disabling extensions. Update the user-facing text to cover plugins as well (or use a generic read-only message).

		// Extension and plugin files cannot be deleted
		if (storage === PromptsStorage.extension || storage === PromptsStorage.plugin) {
			await dialogService.info(
				localize('cannotDeleteExtension', "Cannot Delete Extension File"),
				localize('cannotDeleteExtensionDetail', "Files provided by extensions cannot be deleted. You can disable the extension if you no longer want to use this customization.")
			);