sessions: convert built-in prompts to skills and add UI Integration badge

[joshspicer]

Summary

This PR simplifies the sessions built-in customization architecture by consolidating two parallel discovery systems into one, and adds a "UI Integration" badge to help users understand when overriding a skill affects a UI surface.

What changed

1. Built-in prompts → skills

All 6 built-in prompt files have been converted from vs/sessions/prompts/*.prompt.md to vs/sessions/skills/{name}/SKILL.md with proper SKILL.md frontmatter (name, description). This eliminates the separate built-in prompt discovery system entirely — all built-in customizations are now skills.

Converted skills: act-on-feedback, create-draft-pr, create-pr, generate-run-commands, merge-changes, update-pr

2. Simplified AgenticPromptsService

• Removed BUILTIN_PROMPTS_URI, _builtinPromptsCache, getBuiltinPromptFiles(), discoverBuiltinPrompts()
• listPromptFiles() now early-returns for non-skill types instead of branching
• listPromptFilesForStorage() collapsed to a single conditional
• Removed unused getCleanPromptName import

3. "UI Integration" badge in customizations editor

• Added getSkillUIIntegrations(): ReadonlyMap<string, string> to the IAICustomizationWorkspaceService interface
• Sessions implementation provides mappings for skills with UI connections:
  • act-on-feedback → "Used by the Submit Feedback button in the Changes toolbar"
  • generate-run-commands → "Used by the Run button in the title bar"
• Core VS Code implementation returns an empty map (no-op)
• The list widget looks up skill names against this map and shows a badge + tooltip
• Badge appears on both the built-in skill and any user/workspace override, so users know overriding affects a UI surface

Why

• Simplicity: One discovery system instead of two. Skills already have richer metadata (name validation, description parsing, disableModelInvocation, userInvocable), so prompts were the weaker abstraction.
• Discoverability: Users who override a built-in skill that drives a toolbar button should see that relationship in the customizations editor, not discover it by accident.

Testing

• ✅ TypeScript compilation (tsgo): zero errors
• ✅ Layering check: all 6 layer projects pass
• ✅ Hygiene/pre-commit hook: passed

[Copilot]

Pull request overview

This PR consolidates Sessions “built-in customizations” so they’re discovered exclusively as skills (removing the parallel built-in prompt discovery path), and adds a “UI Integration” badge in the AI Customizations management UI to indicate skills that are invoked directly by UI surfaces.

Changes:

• Convert Sessions bundled built-ins from .prompt.md files into skills/{name}/SKILL.md skills with frontmatter metadata.
• Simplify AgenticPromptsService by removing built-in prompt discovery/caching and making built-ins apply only to skills.
• Add IAICustomizationWorkspaceService.getSkillUIIntegrations() and use it in the customization list to show a “UI Integration” badge + tooltip.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/common/aiCustomizationWorkspaceService.ts	Adds getSkillUIIntegrations() to the workspace service interface.
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationWorkspaceService.ts	Core VS Code implementation returns an empty map (no-op).
src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts	Uses getSkillUIIntegrations() to render a “UI Integration” badge for skills.
src/vs/sessions/contrib/chat/browser/promptsService.ts	Removes built-in prompt discovery and makes built-ins apply only to skills.
src/vs/sessions/contrib/chat/browser/aiCustomizationWorkspaceService.ts	Sessions implementation provides UI-integration mappings for specific built-in skills.
src/vs/sessions/skills/act-on-feedback/SKILL.md	Converted bundled customization to a skill with frontmatter + heading.
src/vs/sessions/skills/create-draft-pr/SKILL.md	Converted bundled customization to a skill with frontmatter + heading.
src/vs/sessions/skills/create-pr/SKILL.md	Converted bundled customization to a skill with frontmatter + heading.
src/vs/sessions/skills/generate-run-commands/SKILL.md	Converted bundled customization to a skill with frontmatter + heading.
src/vs/sessions/skills/merge-changes/SKILL.md	Converted bundled customization to a skill with frontmatter + heading.
src/vs/sessions/skills/update-pr/SKILL.md	Converted bundled customization to a skill with frontmatter + heading.
src/vs/sessions/AI_CUSTOMIZATIONS.md	Updates Sessions customization docs to reflect “built-in skills” + documents the new badge.

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/aiCustomization/aiCustomizationListWidget.ts:1182

• Same issue for disabled skills: uiIntegrations.get(disabledName) uses file.name (frontmatter) when present. If the frontmatter name diverges from the folder name, the UI Integration badge can disappear for the disabled override even though the underlying skill id is still integrated in the UI. Prefer using the folder name (basename(dirname(file.uri))) for the integration lookup.

						const filename = basename(file.uri);
						const disabledName = file.name || basename(dirname(file.uri)) || filename;
						const uiTooltip = uiIntegrations.get(disabledName);
						items.push({