docs(bf-5h6d3): disk pre-flight check — 28G free on /

Split-child of bf-146g8 (build-only decomposition), step 1 of 4.
DISK PRE-FLIGHT ONLY — no build, no code changes.

df -BG --output=avail / = 28G free, above the 20G threshold.
No target/ cleanup needed. Recorded as comment on the bead.

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

notes/bf-5h6d3.md

@@ -0,0 +1,26 @@
+# bf-5h6d3 — Disk Pre-Flight Check
+
+Split-child of bf-146g8 (build-only decomposition), **step 1 of 4**.
+DISK PRE-FLIGHT ONLY — no build, no code changes.
+
+## Result
+
+```
+$ df -BG --output=avail / | tail -1
+ 28G
+```
+
+**Free disk on `/`: 28G** — above the ~20G threshold.
+
+## Action taken
+
+None required. Per the task acceptance criteria, since free space (28G) is
+≥ 20G, no `target/` cleanup was performed. `~/miroir/target` was left
+untouched regardless (not mid-build).
+
+## Acceptance criteria
+
+- [x] GB free on `/` recorded (28G) — see comment 15 on the bead and above.
+- [x] Below-threshold case not triggered (28G ≥ 20G), so no idle
+      other-repo `target/` was cleared.
+- [x] No code changes.