chore(cicd): add daily job to check reference validity

[pjhampton]

Adds a daily CI job that checks all external HTTP/HTTPS links in the skills directory to ensure they remain valid over time.

Motivation

External links can break over time due to:

• Domain changes or site restructuring
• Deprecated documentation pages
• Service shutdowns
• URL typos during authoring

This automated check helps maintain documentation quality by detecting broken links before users encounter them.

When run on #2

$ bun src/check-external-links.ts
Checking external links...
Skill directory: /Users/pjhampton/workspace/agent-skills/skills/clickhouse-best-practices

Found 32 files to scan

Found 13 unique external links

Checked 5/13 links...
Checked 10/13 links...
Checked 13/13 links...

================================================================================
External Links Check Summary
================================================================================

┌─────────────────────────────────────────────────────────────────────────────┐
│ URL                                                      │ Status │ Source  │
├──────────────────────────────────────────────────────────┼────────┼─────────┤
│ https://clickhouse.com/docs                              │ 200 ✓  │ ...data.json │
│ https://clickhouse.com/docs/best-practices               │ 200 ✓  │ README.md │
│ https://clickhouse.com/docs/best-practices/avoid-muta... │ 200 ✓  │ ...delete.md │
│ https://clickhouse.com/docs/best-practices/avoid-opti... │ 200 ✓  │ ...-final.md │
│ https://clickhouse.com/docs/best-practices/choosing-a... │ 200 ✓  │ ...ecycle.md │
│ https://clickhouse.com/docs/best-practices/choosing-a... │ 200 ✓  │ ...rderby.md │
│ https://clickhouse.com/docs/best-practices/minimize-o... │ 200 ✓  │ ...before.md │
│ https://clickhouse.com/docs/best-practices/select-dat... │ 200 ✓  │ ...llable.md │
│ https://clickhouse.com/docs/best-practices/selecting-... │ 200 ✓  │ ...h-size.md │
│ https://clickhouse.com/docs/best-practices/use-data-s... │ 200 ✓  │ ...ndices.md │
│ https://clickhouse.com/docs/best-practices/use-json-w... │ 200 ✓  │ ...to-use.md │
│ https://clickhouse.com/docs/best-practices/use-materi... │ 200 ✓  │ ...shable.md │
│ https://github.com/ClickHouse/ClickHouse                 │ 200 ✓  │ ...data.json │
└──────────────────────────────────────────────────────────┴────────┴─────────┘

Summary: 13 links checked, 13 passed, 0 failed

✓ All 13 external links are valid

[alexkorep]

The script contains a lot of logic, I would consider to add unit tests, maybe as a separate PR?

[pjhampton]

The script contains a lot of logic, I would consider to add unit tests, maybe as a separate PR?

SGTM