[Content] Fetch, normalize, and hash allowlisted Learn sources

[troyscott]

Outcome

Retrieve approved official sources on demand and create deterministic change inputs.

Acceptance criteria

• Only HTTPS Microsoft Learn hosts and approved paths are accepted.
• Redirects, timeouts, and response sizes are bounded and revalidated.
• Raw and normalized SHA-256 hashes are recorded.
• Conditional requests are used when supported.
• Private source snapshots are excluded from Git and public artifacts.