ci: stop disk reclaim from deleting a smoke image mid-test

v1.12.2 run 740: build-variant's `docker system prune -af --volumes`
untagged pi-devbox:smoke-studio 30 ms after smoke-studio loaded it, so
every studio assertion failed and the -studio tags were not published.
All jobs share the runner's Docker daemon, and a smoke image is unused
between smoke-test.sh's one-shot `docker run`s.

- scripts/ci-reclaim-docker.sh: the same reclaim, per object type, but
  never prunes an image labelled se.jordbo.pi-devbox.ci-smoke-image;
  labelled orphans are swept after 6h. `assert-protected IMG` is the
  smoke preflight; `lint WORKFLOW...` rejects raw system/image prunes.
- smoke jobs: label their image, assert it, tag it per run
  (pi-devbox:smoke[-studio]-<run_id>), remove it in an always() step.
- all five reclaim sites call the script; lint.yml runs the guard.
- Job order unchanged: studio jobs still gate only the -studio tags.

Verified on an isolated docker:29-dind (old prune deletes a labelled
image; new reclaim keeps it, removes unlabelled + 1970-created labelled
images, spares an in-use image, no errors); actionlint, shellcheck,
lint-shell, check-workflow-shell, check-doc-drift all clean.

AGENTS.md

@@ -45,6 +45,13 @@ re-brand of opencode-devbox's `pi-only` variant.
   update-description). The `-studio` variant adds independent
   `smoke-studio` + `build-variant-studio` jobs that gate only the
   `-studio` tags (never the core `:latest` release).
+- `scripts/ci-reclaim-docker.sh` — the ONLY way a CI job frees disk. All jobs
+  share the runner's Docker daemon, so a bare `docker system prune -a` in one
+  job deletes the smoke image another job is mid-test on (v1.12.2 run 740 lost
+  its `-studio` tags that way). The script spares images labelled
+  `se.jordbo.pi-devbox.ci-smoke-image`; the smoke jobs apply that label,
+  assert it (`assert-protected`) and remove their own image at the end.
+  `lint.yml` fails on any raw `docker system|image prune` in a workflow.
 
 ## Versioning scheme