ci: bust the re-poisoned cargo cache (v3) + burst-guard the runner prune

This session's push storm refilled the runner to 100% WITHIN the prune timer's 24h window
(it only trims >24h), so a build hit ENOSPC and actions/cache saved a truncated target/ ->
`error[E0463]: can't find crate for shlex` in ci.yml's clippy. Two fixes:

- Bump cargo-target-v2- -> v3- in ci.yml + deb.yml so the poisoned tarball is bypassed (a
  suffix bump can't — restore-keys falls back to the old prefix; same as the v1->v2 fix).
- Harden scripts/ci/docker-prune: run HOURLY (was 6h) with a burst guard — if the disk is
  still >85% after the normal until=12h trim, prune ALL idle images + build cache (in-use
  protected). A fast push-burst can fill 99 GB inside any time window, so the disk-pressure
  trigger, not the age filter, is the real backstop. Applied live on home-runner-1 (reclaimed
  95%->66%) and checked in.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/ci/docker-prune.timer

@@ -1,12 +1,12 @@
-# Runs docker-prune.service every 6h. Persistent=true catches up after downtime.
-# Install: see the header of docker-prune.service.
+# Runs docker-prune.service hourly (the burst guard needs to react within the hour, not every 6h).
+# Persistent=true catches up after downtime. Install: see the header of docker-prune.service.
 
 [Unit]
-Description=Run docker-prune every 6h (CI runner disk hygiene)
+Description=Run docker-prune hourly (CI runner disk hygiene + burst guard)
 
 [Timer]
-OnCalendar=*-*-* 00/6:00:00
-RandomizedDelaySec=600
+OnCalendar=hourly
+RandomizedDelaySec=300
 Persistent=true
 
 [Install]