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

[vreshch]

Run 1 (2026-07-31) of the estate-wide Friday release train shipped zero releases: GitHub fired the crons 60-115 minutes late, and the exact-hour Prague gate self-skipped every run green (no failure signal, just silent no-ops).

This moves server-memory (L1) off the double-cron + Prague-hour-gate pattern to a single UTC cron - '0 20 * * 5' - with the gate job deleted entirely. A late-firing cron now just means the train departs late instead of being gated away.

• Schedule: single cron: '0 20 * * 5' (L1 slot, one hour after L0's 19:00 UTC so it picks up freshly published @agentage L0 packages)
• Removed the prague-gate job and all needs/if: needs.prague-gate.outputs.go == 'true' references on downstream jobs
• workflow_dispatch unchanged - manual trigger still works
• Internal @agentage/* dependency bump (npm-check-updates) logic untouched
• Header/inline comments updated to reflect single UTC cron + L0 19:00 UTC timing
• .github/dependabot.yml shifted to Friday 16:00 UTC (timezone: 'Etc/UTC') from Prague time

Part of the estate-wide UTC shift.