feat: group chat extension customizations under 'Built-in' in managem…

[joshspicer]

closes #303584

…ent editor

Items contributed by the default chat extension (GitHub Copilot Chat) are now shown under a 'Built-in' group header instead of 'Extensions' in the Chat Customizations editor. This applies to agents, skills, prompts, and instructions — matching the existing pattern used by the MCP list widget.

The grouping is determined at the UI layer using IProductService to identify the chat extension ID, setting groupKey on matching items. No data model changes needed.

• Move BUILTIN_STORAGE/AICustomizationPromptsStorage to common layer
• Add BUILTIN_STORAGE to VS Code harness filter sources
• Add isChatExtensionItem() helper using productService.defaultChatAgent
• Set groupKey: BUILTIN_STORAGE for items from the chat extension
• Add tests for builtin source filtering
• Update AI_CUSTOMIZATIONS.md spec

[Copilot]

Pull request overview

Updates the Chat Customizations management editor to visually group items contributed by the default chat extension (GitHub Copilot Chat) under a “Built-in” header (instead of “Extensions”), aligning with existing MCP list grouping patterns.

Changes:

• Move BUILTIN_STORAGE / AICustomizationPromptsStorage into the common AI customization workspace service and re-export from existing entrypoints.
• Extend the core Local harness’ storage source list to include builtin, and add tests ensuring the storage-source filter respects it.
• Add UI-layer detection (IProductService.defaultChatAgent.chatExtensionId) to set groupKey: BUILTIN_STORAGE for items coming from the default chat extension.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/test/browser/aiCustomization/applyStorageSourceFilter.test.ts	Adds coverage for builtin source behavior in applyStorageSourceFilter.
src/vs/workbench/contrib/chat/common/aiCustomizationWorkspaceService.ts	Defines AICustomizationPromptsStorage and BUILTIN_STORAGE in the common layer.
src/vs/workbench/contrib/chat/browser/aiCustomization/customizationHarnessService.ts	Includes BUILTIN_STORAGE in the core Local harness sources.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationManagement.ts	Re-exports BUILTIN_STORAGE and the storage union type from common.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts	Implements built-in grouping for default chat extension items via groupKey.
src/vs/sessions/contrib/chat/common/builtinPromptsStorage.ts	Re-exports the shared built-in storage discriminator/type for sessions compatibility.
src/vs/sessions/AI_CUSTOMIZATIONS.md	Documents the new built-in grouping behavior and updated source list.

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts:1056

• In the Skills section, this now always calls listPromptFiles(PromptsType.skill, ...) to build an extensionId lookup. That can be noticeably more expensive than needed (it enumerates all storages) and is a regression from the previous behavior where the call only happened when disabled skills needed to be included. Consider building the lookup via listPromptFilesForStorage(PromptsType.skill, PromptsStorage.extension, ...) (and only when there are any extension skills), keeping the full listPromptFiles call inside the disabled-skills path if still required for that.

			for (const agent of agents) {
				const filename = basename(agent.uri);
				const isChatExt = agent.source.storage === PromptsStorage.extension && this.isChatExtensionItem(agent.source.extensionId);
				items.push({
					id: agent.uri.toString(),
					uri: agent.uri,
					name: agent.name,