sessions: built-in skills (#301607)

* sessions: add built-in skills infrastructure

Add a 'built-in skills' concept mirroring the existing 'built-in prompts'
pattern, so that skills bundled with the Sessions app are always available.

- Create src/vs/sessions/skills/ directory for bundled SKILL.md files
- Bundle vs/sessions/skills/**/SKILL.md in build config
- AgenticPromptsService: discover and parse built-in skills from the
  bundled directory, override findAgentSkills() to merge them at lowest
  priority (user/workspace skills override by name), extend listPromptFiles
  and listPromptFilesForStorage for skill type
- Tree view: handle BUILTIN_STORAGE group for skills category

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* sessions: add built-in 'update-skills' skill

First built-in skill that guides the agent to capture major repository
learnings by creating or updating skills and instructions. Triggered
when the user says 'learn!' or when significant reusable knowledge is
discovered during a session.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* sessions: support edit-and-save-to-override for built-in skills

Extend the built-in prompt save-to-override flow to also work for
built-in skills. When editing a built-in skill, users can save it to
workspace or user location, which overrides the built-in version.

- Gate the in-memory editing session on both prompt and skill types
- Resolve save target directories using the actual prompt type
- Preserve skill directory structure ({name}/SKILL.md) when saving
- Generalize UI labels from 'prompt override' to 'override'

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* sessions: address PR review feedback for built-in skills

- Sanitize built-in skill name/description (strip XML tags, truncate)
  to match the same safety applied to other skill sources
- Fix JSDoc: all sources override built-ins, not just user/workspace
- Remove unnecessary 'as unknown as PromptsStorage' cast in tree view

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* sessions: refine editor options types for accessibility and font settings

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

build/gulpfile.vscode.ts

@@ -100,6 +100,7 @@ const vscodeResourceIncludes = [
 	// Sessions
 	'out-build/vs/sessions/contrib/chat/browser/media/*.svg',
 	'out-build/vs/sessions/prompts/*.prompt.md',
+	'out-build/vs/sessions/skills/**/SKILL.md',
 
 	// Extensions
 	'out-build/vs/workbench/contrib/extensions/browser/media/{theme-icon.png,language-icon.svg}',