Add Posit Assistant option for install

[wklimowicz]

Adding a shim to here for Posit Assistant would be great:

ai-dev-kit/install.sh

Lines 1716 to 1718 in b059fd0

	copilot) echo "$base_dir/.github/skills" ;;
	codex) echo "$base_dir/.agents/skills" ;;
	gemini) echo "$base_dir/.gemini/skills" ;;

At first I thought I'd need to install for another agent, and then copy and paste the skills, but posit assistant supports ~/.agents/ out of the box:

User-level — ~/.posit/assistant/skills/{skill-name}/ or ~/.agents/skills/{skill-name}/ — available across all projects

Project-level — .posit/assistant/skills/{skill-name}/ or .agents/skills/{skill-name}/ in your project root — scoped to a single project

https://assistant.posit.co/docs/features/skills/