[mohitj1988]

Enable the nightly QA, testing one major per weeknight instead of everything (a full run is 216 Jenkins builds, one major is 30-45 builds and takes about 1h20m):

Mon PG 14 (30 builds), Tue PG 15 (36), Wed PG 16 (37), Thu PG 17 (39),
Fri PG 18 (45); Sat/Sun free for re-runs and maintenance; PG 19 on demand.

• .github/nightly-qa-schedule.yml: weekday -> major, the one place to change the plan.
• .github/scripts/nightly_plan.py (new job 'plan'): tonight's major (schedule or the new 'major' input: today, 14-19, all). A scheduled run is skipped when nothing under root/ changed since the last successful run of the SAME major, found by a 'nightly-major-' artifact each run uploads; comparing with any last run would let a quiet Monday hide a change from Tuesday's major. Manual runs always run. A package refresh job runs 4 hours earlier, so component changes reach OBS before the check.
• list_qa_matrix.py: QA_MAJOR keeps one major's combos: its own projects (ppg:staging:...) plus shared-image entries named -pg (component images) or -to- (upgrades, counted on the major they upgrade into).
• obs-nightly-qa.yml: schedule 23:00 UTC Mon-Fri; never cancel a running nightly; at most 20 jobs at once; 6-hour job timeout.
• tests: selection rule, schedule file, plan decisions (including the same-major comparison), and that every QA build of the tree runs on exactly one scheduled night.