Agents welcome: preview/import API for VS Code theme importer

[sandy081]

Refactors the VS Code theme importer service with preview/import separation and addresses PR feedback from #313333.

Theme Importer API changes

• previewVSCodeTheme() — installs the providing extension temporarily from the host's extensions directory, applies the theme, and returns an IDisposable that uninstalls the extension on dispose
• importVSCodeTheme() — installs from host location (immediate), copies the extension to the Agents app's own extensions directory, then replaces the install from the copied location (permanent)

Walkthrough integration

• Selecting the VS Code theme option calls preview (temporary)
• Clicking Continue calls import (permanent copy)
• Pressing Escape disposes the preview (uninstalls temp extension)

PR #313333 feedback addressed

• Compare both id and label in theme extension matching
• Move VS Code theme radio inside the radiogroup for accessibility
• Add touch/tap support via Gesture.addTarget + TouchEventType.Tap
• Add touch-action: manipulation CSS to eliminate 300ms tap delay
• Fix JSDoc comments for accuracy

[Copilot]

Pull request overview

This PR refactors the Agents welcome flow to add a theme-selection step that can preview and then permanently import the user’s active VS Code theme by temporarily installing (preview) and then copying/installing (import) the providing theme extension from the host VS Code installation.

Changes:

• Introduces IVSCodeThemeImporterService with previewVSCodeTheme() (temporary install) and importVSCodeTheme() (permanent copy + install).
• Extends the Sessions walkthrough to include a theme step with onboarding themes plus an optional “Use My VS Code Theme” option, including tap support and new CSS.
• Adds host-environment path APIs (hostUserHome, hostUserRoamingDataHome, hostExtensionsHome) and updates embedded-profile defaults to use them.

Show a summary per file

File	Description
src/vs/sessions/sessions.common.main.ts	Ensures the theme importer singleton is registered in the Sessions app entrypoint.
src/vs/sessions/services/vscode/common/vsCodeThemeImporter.ts	New service for resolving, previewing, and importing the host VS Code theme/extension.
src/vs/sessions/contrib/welcome/browser/welcome.contribution.ts	Routes sign-in completion to the new theme step instead of completing the walkthrough immediately.
src/vs/sessions/contrib/welcome/browser/sessionsWalkthrough.ts	Adds the theme-selection step UI and wires preview/import behavior.
src/vs/sessions/contrib/welcome/browser/media/sessionsWalkthrough.css	Styling for the new theme step and VS Code-theme option, including touch-action improvements.
src/vs/platform/userDataProfile/electron-main/userDataProfile.ts	Updates host agent-plugins path resolution for embedded mode.
src/vs/platform/environment/node/environmentService.ts	Removes node-only host roaming home getter (moved to common base class).
src/vs/platform/environment/common/environmentService.ts	Adds host path getters (hostUserHome, hostUserRoamingDataHome, hostExtensionsHome).
src/vs/platform/environment/common/environment.ts	Extends IEnvironmentService with host path properties used by Sessions features.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/sessions/contrib/welcome/browser/sessionsWalkthrough.ts:458

• The preview disposable is always disposed when the step tears down, even after a successful importVSCodeTheme(). Because the preview disposable uninstalls by extension identifier, this can unintentionally uninstall the newly imported (permanent) extension right after import. Dispose and clear the preview installation before starting the permanent import (or otherwise ensure the preview cleanup doesn't uninstall once the user commits the import).

		stepDisposables.add(addDisposableListener(continueBtn, EventType.CLICK, async () => {
			if (isVSCodeThemeSelected) {
				await this.vsCodeThemeImporter.importVSCodeTheme();
			}

• Files reviewed: 4/4 changed files
• Comments generated: 3