#25943 ci(nightly-next): recover stale feeds beside older builds

The NEXT feed watcher now checks every five minutes and keeps the 25-minute recovery threshold below the 30-minute freshness bound. When the feed is stale, an older nightly build no longer suppresses the newest candidate; only an active promotion request or a nightly already building that exact tip blocks the automatic tick. The tick continues to use request-nightly-next.sh's shared tree-ready check, so stale or unpublished candidates are skipped.

Validation:

• python3 tests/test_nightly_next_feed_watch.py
• bash scripts/cmux-next/tests/request-nightly-next.test.sh
• git diff --check

No hand promotion, dispatch, cancellation, or nightly.yml changes are included.

Agent-Session: https://cmux.com/deeplink/agent?id=aid-f67d84e355a5bb96dc66

---

^{Need help on this PR? Tag @codesmith-bot with what you need.}