A pure read picks the crew pulls with no push for 7 days, oldest first, with the note that folds each back into its issue #6521

Part of #5930, section 3: "Close pulls stale more than 7 days and fold them back into their issues", a decision a person takes, so this only proposes.

Done:

• [ ] New src/flow/stale_fold.ts exports staleFold(pulls, now), where pulls are { number, issueNumber, branch, lastPushAt, approved } plain data. It returns the ones with no push for 7 days, oldest first, leaving out approved ones, each with note: "Folded back: pull #N had no push for D days; its branch <branch> keeps the work."
• [ ] A pull with no issue is listed with issueNumber: null and no note.
• [ ] Tests: the 7-day edge, the order, approved pulls left out, the note's words.

Files: src/flow/stale_fold.ts, test/flow-stale-fold.test.ts.

Before pushing: bun x tsc --noEmit and the new test.