TASK: EngStack disk reclaim (handoff sequence step 1)

Objective: Free disk on this workstation (/ is 95%, 14G free) by pruning DANGLING Docker build cache only. Handoff doc: ~/.coordinate/CTOHandoff.md.

Context:

/home/TSGCTO/projects/TSYSDevStack-RandD/EngStack (build scripts, TODO.md)
docker system df today: Build Cache 109GB total, 42.7GB reclaimable; Images 108.2GB, 44.71GB reclaimable (do NOT touch yet)

Constraints:

NEVER stop/remove running containers (26 SupportStack-Local demos live).
Allowed: docker builder prune -f (dangling cache only). No -a, no system prune, no image prune, no volume prune.
If free space still <20% after, write QUESTION-*.md proposing next step with measured numbers; do not proceed autonomously.

Deliverable(s): REPORT-*.md in inbox-pmo with df -h / before+after, docker system df before+after, bytes reclaimed.

Priority: P1