Audit external links in non-legacy documentation

Background

The handbook depends on official Git, GitHub, OpenAI, Anthropic, Atlassian, and company practice references. External links should stay fresh, especially outside the archived legacy section.

Scope

• Review external links in active documentation directories.
• Skip 09-resources/legacy/ unless a link is referenced from an active guide.
• Prefer official documentation, source repositories, release notes, and primary sources.
• Replace outdated links when a stable official URL exists.
• Mark outdated resources clearly when keeping them for historical context.

Done When

• Broken or redirected links in active docs are fixed or documented.
• No historical resource is presented as the primary learning path.
• python3 scripts/check-docs.py passes.
• git diff --check passes.