Prepare v0.5.0 release

[dacharyc]

Implement the following checks:

• rendering-strategy (net new, reflected in v0.2.0 of the Agent-FriendlyDocs Spec)
• tabbed-content-serialization
• section-header-quality
• markdown-content-parity
• llms-txt-freshness
• auth-alternative-access

Add the following new functionality:

• Sampling strategy (a new optional --sampling flag) to enable a broader range of use cases

Fix the following bugs:

• Cloudflare and Supabase docs showed no markdown content due to a bug in the llms.txt markdown discovery logic. Added progressive disclosure llms.txt file path walking support to get to markdown URLs.
• The tool was showing that Cloudflare did not support content negotiation, but it was a false fail due to an issue in the logic - fixed here