ci: run release train on plain UTC cron, drop Prague hour gate

[vreshch]

Run 1 (2026-07-31) shipped zero releases: GitHub fired the crons 60-115 min late, and the exact-hour Prague gate self-skipped every run green (both cli runs landed in hour 23, not the target 22). The gate never got a chance to fire.

This moves to a single UTC cron ('0 21 * * 5', the L2 slot) with the Prague/DST gate deleted entirely - a late cron now means the train departs late instead of being gated away. workflow_dispatch is unchanged (still bypasses nothing, since there's no gate left to bypass). Dependabot schedule shifted to Friday 16:00 UTC (Etc/UTC) to match.

Last-tag anchoring logic (stale v0.24.x tags) is untouched.

Part of the estate-wide UTC shift.

[github-actions]

🎉 PR Validation ✅ PASSED

Commit: 8ab30ec735529e59ff6c24364c776c437be4386b
Branch: feature/utc-cron

Checks:

• ✅ Release guard (no version/changelog changes)
• ✅ Dependencies installed
• ✅ Type check passed
• ✅ Linting passed
• ✅ Format check passed
• ✅ Tests + coverage passed
• ✅ Build successful

Ready to merge! ✨

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⏰ Generated at: 2026-07-31T23:56:32.336Z