upgrade-all: pin weekly slot (Sat 03:00 UTC) + defer activation until cc-ci is built

Operator: don't run the weekly upgrade-all while the build loops are still
constructing cc-ci (shared-host contention). Activate the Sat 03:00 UTC
(0 3 * * 6) cron only once the build is complete; on-demand until then.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.claude/skills/upgrade-all/SKILL.md

@@ -85,6 +85,16 @@ End with the report path and a reminder that **nothing was merged**.
 - **Never merges**; failures/ skips are surfaced and retried next week — safe to re-run anytime.
 
 ## Cron
-Designed for a weekly Claude Code scheduled task (configured separately) that invokes `/upgrade-all`
-in `/srv/cc-ci`. Re-running is idempotent: already-current recipes report `SKIPPED — up-to-date`;
-recipes with an open PR for the same branch report the existing PR rather than duplicating it.
+Designed for a weekly Claude Code scheduled task that invokes `/upgrade-all` in `/srv/cc-ci`.
+
+**Agreed schedule:** **Saturday 03:00 UTC** (`0 3 * * 6`) — low-traffic weekend window, PRs waiting by
+Monday.
+
+**Activation trigger (operator, 2026-05-29):** do NOT activate while the build loops are still
+constructing cc-ci — it would contend with them for the shared host. **Activate this weekly cron only
+once the cc-ci build is complete (loops finished / cc-ci stable.)** Until then it's run manually /
+on-demand. When activating, create a scheduled task that runs `/upgrade-all` in `/srv/cc-ci` at
+`0 3 * * 6` UTC.
+
+Re-running is idempotent: already-current recipes report `SKIPPED — up-to-date`; recipes with an open
+PR for the same branch report the existing PR rather than duplicating it.