sessions: convert built-in prompts to skills and add UI Integration badge (#305347)

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

- Move all 6 built-in prompts from vs/sessions/prompts/ to
  vs/sessions/skills/{name}/SKILL.md with proper frontmatter
- Remove the built-in prompt discovery system (discoverBuiltinPrompts,
  getBuiltinPromptFiles, BUILTIN_PROMPTS_URI) from AgenticPromptsService
- Simplify listPromptFiles/listPromptFilesForStorage to only handle
  skills as the built-in type
- Add getSkillUIIntegrations() to IAICustomizationWorkspaceService
  interface, returning a map of skill names with UI surface connections
- Sessions implementation maps act-on-feedback (Submit Feedback button)
  and generate-run-commands (Run button) to tooltips
- Show 'UI Integration' badge in the customizations editor for skills
  that drive UI surfaces, including user overrides of those skills
- Update AI_CUSTOMIZATIONS.md to reflect the simplified architecture

Co-authored-by: Copilot <[email protected]>

* address review feedback: stable folder-name lookup, safe builtin fallback, shared empty map

- Use basename(dirname(skill.uri)) instead of skill.name for UI
  integration lookup so badge persists even if frontmatter name changes
- Return [] for BUILTIN_STORAGE on non-skill types instead of
  delegating to super (which would throw)
- Use a shared static empty map in the core VS Code implementation to
  avoid repeated allocations

Co-authored-by: Copilot <[email protected]>

* add built-in skills with UI integration badges for sessions

* add missing UI Integration mappings for all toolbar-connected skills

Add create-pr, create-draft-pr, update-pr, merge-changes, and commit
to the skill UI integrations map. These are all triggered by buttons
in the Changes toolbar.

Co-authored-by: Copilot <[email protected]>

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Co-authored-by: Copilot <[email protected]>