Dxcdt 1557/install claude skills with auth0 cli

[KartikJha]

🔧 Changes

Introduces a new internal/ai/skills package that provides an agent registry and detection layer for AI coding agents. This
is the foundation for the auth0 ai skills install command, enabling the CLI to discover which agents are present on a user's
machine and resolve the correct skills directory for each.

New types and functions:

• AgentConfig — describes a single AI coding agent with its ID, display name, global/project skills directories, and
  detection hints (filesystem markers and binary names)
• AgentConfig.IsInstalled() — returns true if any marker path exists on disk or any binary is found in $PATH
• SupportedAgents — registry of 20 agents: Claude Code, Cursor, GitHub Copilot, Gemini CLI, Roo Code, Goose, OpenCode,
  Codex, Windsurf, Continue, Amp, Junie, Kiro CLI, Cline, Augment, AiderDesk, Warp, OpenHands, Trae, and a Universal fallback
• DetectedAgents() — returns the subset of SupportedAgents that are installed, plus the Universal agent; result is cached
  via sync.Once
• FastPriorityAgents() — returns detected agents sorted by priority order (Claude Code → Cursor → GitHub Copilot → Gemini
  CLI → others → Universal last), used by the --fast install mode

Cleanup:

• Removed three dead flag variables (qsType, qsAppName, qsPort) from quickstarts.go that were no longer referenced
• Minor comment capitalisation fixes in quickstart_detect.go and quickstart.go

🔬 Testing

All new behaviour is covered by unit tests in internal/ai/skills/agent_test.go:

• TestIsInstalled — marker path existence, binary lookup, empty-string skipping, fallback from marker miss to binary hit
• TestSupportedAgents — non-empty registry, unique IDs, non-empty skill dirs, presence of universal, agents with nil
  detection markers
• TestDetectedAgents — universal always present, deterministic caching, all results are subsets of SupportedAgents
• TestFastPriorityAgents — universal last, no duplicates, all detected agents present, priority ordering invariant, length
  equality

Run with:
go test ./internal/ai/skills/...

No external services or credentials are required; IsInstalled checks are exercised using t.TempDir() and t.Setenv("PATH", ...).