Add URL verification script and fix broken URLs

[thodges-gh]

Description

Adds a URL validation script and an advisory CI workflow that check every http(s) link referenced
in the skills' markdown, and fixes the broken links it surfaced.

• .github/scripts/check-urls.sh — extracts every URL from skill *.md files (skipping
  placeholders like localhost/example.com and vendored node_modules), checks each with curl,
  and prints a file:line report grouped into three tiers: OK (2xx/3xx), warn
  (401/403/405/429 — reachable but refused), and broken (404/5xx and connection failures, which
  exit non-zero). Includes a live progress counter, GitHub ::error annotations in CI, and a
  moderate default concurrency (8) so it doesn't trip docs.chain.link rate limiting. A full scan runs
  in ~40s.
• .github/url-check-endpoints.txt — a reachability-only allowlist for API/RPC/WebSocket
  endpoints (e.g. svr-bid-endpoint.chain.link) that legitimately return 404 to a plain GET.
  Matching URLs pass as long as the server completes a connection; only a true connection failure
  (code 000) is treated as broken.
• .github/workflows/url-check.yml — runs the script on PRs touching *-skill/** (plus manual
  dispatch). Advisory only: continue-on-error means it never blocks a merge; broken links surface
  as inline annotations and in the job summary.
• Broken links fixed (all found by the script): CRE cre-prediction-market-demo →
  cre-gcp-prediction-market-demo; CRE …/reference/cre-cli → …/reference/cli.md; ACE
  …/tree/main/script → …/tree/main/scripts and removed the dead
  raw.githubusercontent.com/.../chainlink-ace/main/ base URL. Bumped chainlink-cre-skill to
  0.0.11 and chainlink-ace-skill to 0.0.6

Justification

The skills' usefulness depends on their reference URLs being correct — agents fetch and cite these
links at runtime, so a dead link silently degrades answers or sends users to a 404. These links rot
over time (repos rename paths, docs restructure) with no signal until someone trips over one by
hand, which is exactly how the original cre-prediction-market-demo break was caught. A single
scan across all skills turned up several more stale links beyond the reported one. This makes the
check fast and repeatable so broken URLs are caught on the PR that introduces them — without
blocking merges, since link availability can be transient or depend on external services.

[github-actions]

👋 thodges-gh, thanks for creating this pull request!

To help reviewers, please consider creating future PRs as drafts first. This allows you to self-review and make any final changes before notifying the team.

Once you're ready, you can mark it as "Ready for review" to request feedback. Thanks!

[rgottleber]

LGTM