feat(catalog): add pup commands for the Datadog agentic skills API

Adds a `pup catalog` command group that wraps the public routes of the
agentic skills API so an AI assistant can run `pup catalog …` instead of
raw HTTP calls to discover skills and record sessions. (Distinct from
`pup skills`, which installs skill files onto a local assistant.)

Commands (all hit the existing /api/v2/onboarding/* routes):
- `pup catalog list [--org-id N]` → GET /api/v2/onboarding/skills
- `pup catalog get <skill_id> [--intent …] [--onboarding-run-id …] [--org-id N]`
  → GET /api/v2/onboarding/skills/{skill_id}?format=md, printed as raw markdown
- `pup catalog sessions create --session-id … --skill-id … --summary … --status …
  [--org-id N]` → POST /api/v2/onboarding/sessions

Details:
- `get` always requests the markdown representation (frontmatter + body) —
  what an assistant consumes to follow a skill — so the format isn't exposed
  as a flag.
- skill_id is percent-encoded before path interpolation so IDs with reserved
  characters can't rewrite the route.
- Auth is opportunistic: these routes are OpenAuth, so credentials are
  attached when present but never required.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tedkahwaji