feat: make skills invokable as slash commands in the TUI

- Add Skill.content() method to load skill template content from SKILL.md files
- Modify Command.list() to include skills as invokable commands
- Add 'skill' boolean property to Command.Info schema
- Update autocomplete to show skills with (Skill) label in slash commands
- Regenerate SDK to include skill property in Command type

packages/sdk/js/src/v2/gen/types.gen.ts

@@ -2117,6 +2117,7 @@ export type Command = {
   agent?: string
   model?: string
   mcp?: boolean
+  skill?: boolean
   template: string
   subtask?: boolean
   hints: Array<string>