feat(install): system-wide low-disk login warning via /etc/profile.d

Add etc/profile.d/disk-usage-warning.sh (POSIX sh) that warns in bold
red at login when / or /home cross 85% usage. Deployed system-wide via
install_disk_warning() (sudo install -D -m 0644), gated inside the
apt-get block since df --output=pcent and /etc/profile.d are GNU/Debian
conventions absent on macOS. Idempotent and re-runnable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CN1KSmsuLG6TxSeN5m8xvM

.claude/memory/decisions.md

@@ -33,3 +33,10 @@ Implemented install.sh `setup_remote_desktop` + `ensure_rdp_credentials`. Connec
 Alts rejected: (a) force Xorg GDM + xrdp — sacrifices Wayland desktop, fragile; (b) VNC (wayvnc) —
 RDP preferred (mstsc native on Win client, TLS); (c) g-r-d user "Desktop Sharing" mode — shares
 existing local session, wanted independent headless login. See LRN-004, BLK-004. Status: done.
+
+## BDR-006 — Disk-usage login warning deployed system-wide to /etc/profile.d
+2026-06-24. New `etc/profile.d/disk-usage-warning.sh` (POSIX sh, bold-red warn when / or /home ≥85%)
+deployed via `sudo install -D -m 0644` to `/etc/profile.d/` from `install_disk_warning()`, gated in
+the apt-get Linux block (see LRN-005). Alt rejected: per-user append to `~/.bashrc` — wanted the warn
+for EVERY login account on the box, not just the installing user, so system-wide profile.d won. Known
+limit: login-shell scope only (non-login terminals miss it). Status: done.