Usage report: precise missing-data messages under a budget cut #133

Follow-ups from the final review of #129 (all minor; the report works on live data: full collection in about 7 minutes without hitting its budget).

• When the collection budget runs out, repositories whose run inventory never started still report "older runs may be missing". Report "Run inventory not started for X; all runs are missing" when no window completed, and otherwise name the cursor ("runs created before are missing"). Add a test for the not-started case.
• Only first-page run listings get global newest-first priority; continuation pages queue behind requests already reserved for older runs. Give continuation pages the same priority.
• The release notes for Improve work-record checks and Actions usage reporting #129 don't mention the new alert line for private hosted usage above 300 weighted minutes a week.

Status

State: In review
Scope: Precise budget-cut run-inventory messages and newest-first scheduling for job continuation pages.
PR: #146 (open, no-release)
Decisions: The 300-weighted-minute alert is already mentioned in #129; this is noted in the PR comment, not added to the release-note body.
Remaining: GitHub Actions verification and owner review/merge. No merge was performed; keep this issue open until the PR is delivered.