Function runFetchCorpus

Fetch raw PR comment bundles for the most recent merged/closed PRs in repo that haven't already been processed. Returns a serializable corpus for the host's extract-learnings prompt.

Filters applied at the CLI layer (not the fetcher):

• Only PRs in the requested repo
• PRs older than 12 months are dropped (recency cliff)
• PRs with commentsFetchedAt already set are skipped unless forceRefetch
• Capped at limit (default 5, max 10)

Stamps commentsFetchedAt on every PR that was successfully fetched.