Move Knowledge Store source cards to sources/ - #667

What changed

• Put new Knowledge Store source cards in sources/ and use the card's source: to locate the original. Keep both the existing and new card layouts readable.
• Preserve old citation URLs with committed Markdown redirects when a card moves. Resolve those redirects for source checks and validate both the target and the clickable link.
• Verify source-card and legacy redirect visibility in Git, and document the move with a migrated fixture.

Closes #666.

How it is verified

• git diff --check, the notation documentation check, and its 103 unit tests pass locally.
• The targeted intake-profile tests cover the new source directory, legacy cards and redirects, and ignored cards.
• The Knowledge Store integration tests exercise new, legacy, and migrated citations, including broken redirects and confidence checks.

Checklist

• Based on main and limited to one concern.
• Commit signed off for DCO.
• No internal work-item references.