k8s-version-upgrade: switch detection cron from weekly to daily

Was `0 12 * * 0` (Sun 12:00 UTC) — patch releases waited up to 6 days
before the chain picked them up. Now `0 12 * * *` (daily 12:00 UTC,
still outside kured's 02:00-06:00 London window). Concurrency is
bounded by Forbid + deterministic job-name idempotency (the detection
job exits early if a preflight Job for the same target already exists),
so back-to-back days can't pile up parallel runs.

- stacks/k8s-version-upgrade/main.tf: var.schedule default + rationale comment
- scripts/upgrade_state.sh: rename next_sunday_noon_utc -> next_daily_noon_utc
  (now returns "Tue 2026-05-19 12:00 UTC" form); change "(Sun cron)" label
  to "(daily cron)"
- .claude/skills/upgrade-state/SKILL.md: cadence column + frontmatter

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

stacks/k8s-version-upgrade/main.tf

@@ -26,7 +26,12 @@
 
 variable "schedule" {
   type    = string
-  default = "0 12 * * 0" # Sunday 12:00 UTC — outside kured window
+  # Daily 12:00 UTC — outside kured window (kured runs 02:00-06:00
+  # London). Was weekly Sunday until 2026-05-18; daily picks up upstream
+  # patch releases the same day they land. Concurrency is bounded by the
+  # CronJob's Forbid policy + Job-name idempotency (the detection job
+  # skips spawning a preflight Job if one already exists).
+  default = "0 12 * * *"
 }
 
 variable "enabled" {