Throttle scheduled cron to every 15 minutes

[santicomp2014]

Summary

• Change cron in pr_website.yml, reading.yml, support_kb.yml, eng_annotations.yml from * * * * * to */15 * * * *.
• Stops the Dead Man's Snitch hourly alerts that have been firing in #eng-incidents (e.g. #pr-website, #support-kb, #reading).

Why

GitHub Actions does not reliably honor * * * * * schedules. Observed cadence today on pr-website was ~25-70 min between runs (e.g. 03:59 UTC → 05:09 UTC = 70 min gap), which exceeds the Dead Man's Snitch hourly window and triggers a "missing" alert even though every individual run succeeds. All recent runs are green — the h search API is fine; this is purely a cron-throttling issue on GitHub's side.

*/15 * * * * expresses what GitHub actually delivers, so hourly snitches stop flapping.

Test plan

• Merge and watch the 4 workflows for ~2 hours — runs should land every ~15-45 min as before, but no "snitch missing" alerts in #eng-incidents.
• If alerts still fire, either loosen the snitch interval on deadmanssnitch.com or drop to */30 * * * *.

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