feat: add hideGenerateButton and workspaceSubpaths to customization provider metadata

Closes parity gap between static harness descriptors (setting OFF) and
extension-contributed providers (setting ON). Without these properties,
extension harnesses always showed the AI generation button and allowed
file creation in any workspace directory.

- Add hideGenerateButton to ChatSessionCustomizationProviderMetadata
- Add workspaceSubpaths to ChatSessionCustomizationProviderMetadata
- Plumb both through DTO and mainThread to IHarnessDescriptor

Author: joshspicer

src/vs/workbench/api/browser/mainThreadChatAgents2.ts

@@ -677,6 +677,8 @@ export class MainThreadChatAgents2 extends Disposable implements MainThreadChatA
 			label: metadata.label,
 			icon: metadata.iconId ? ThemeIcon.fromId(metadata.iconId) : ThemeIcon.fromId(Codicon.extensions.id),
 			hiddenSections,
+			hideGenerateButton: metadata.hideGenerateButton,
+			workspaceSubpaths: metadata.workspaceSubpaths ? [...metadata.workspaceSubpaths] : undefined,
 			getStorageSourceFilter: () => ({
 				// Extension-provided harnesses manage their own items via the provider,
 				// so we show all sources for storage-filter-based flows.

src/vs/workbench/api/common/extHost.protocol.ts

@@ -1682,6 +1682,8 @@ export interface IChatSessionCustomizationProviderMetadataDto {
 	readonly label: string;
 	readonly iconId?: string;
 	readonly unsupportedTypes?: readonly string[];
+	readonly hideGenerateButton?: boolean;
+	readonly workspaceSubpaths?: readonly string[];
 }
 
 export interface IChatSessionCustomizationItemDto {

src/vs/workbench/api/common/extHostChatAgents2.ts

@@ -672,6 +672,8 @@ export class ExtHostChatAgents2 extends Disposable implements ExtHostChatAgentsS
 			label: metadata.label,
 			iconId: metadata.iconId,
 			unsupportedTypes: metadata.unsupportedTypes?.map(t => typeConvert.ChatSessionCustomizationType.from(t)),
+			hideGenerateButton: metadata.hideGenerateButton,
+			workspaceSubpaths: metadata.workspaceSubpaths ? [...metadata.workspaceSubpaths] : undefined,
 		};
 
 		this._proxy.$registerChatSessionCustomizationProvider(handle, chatSessionType, metadataDto, extension.identifier);

src/vscode-dts/vscode.proposed.chatSessionCustomizationProvider.d.ts

@@ -63,6 +63,25 @@ declare module 'vscode' {
 		 * when this provider is active.
 		 */
 		readonly unsupportedTypes?: readonly ChatSessionCustomizationType[];
+
+		/**
+		 * When `true`, the "Generate with AI" sparkle button is replaced
+		 * with a plain "New" button for manual file creation.
+		 *
+		 * Use this when the provider's backend does not support
+		 * AI-generated customization scaffolding.
+		 */
+		readonly hideGenerateButton?: boolean;
+
+		/**
+		 * Workspace sub-paths that this provider recognizes for file creation.
+		 * When set, the directory picker for new customization files only
+		 * offers workspace directories under these sub-paths
+		 * (e.g. `['.claude']` for Claude, `['.github', '.copilot']` for CLI).
+		 *
+		 * When omitted, all workspace directories are available.
+		 */
+		readonly workspaceSubpaths?: readonly string[];
 	}
 
 	/**