fix: bound the sweep issue body to the GitHub limit

[abrichr]

The first scheduled sweep died at createIssue: GraphQL rejected the body over 65536 characters, because the org holds ~50 stranded branches and the table was unbounded.

• Cap the remote table at 40 oldest rows with a per-repo summary of the rest.
• Hard-guard the final body at 60000 chars with an explicit truncation note.
• Tests: cap fires and hides nothing silently; a pathological wide fixture stays under the API limit.

Verified against live data: body 6370 chars, 40 shown, +10 summarized.