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.md

@@ -22,6 +22,7 @@ vim/autoload/         pathogen loader (committed)
 vim/colors/           molokai colorscheme (committed)
 bash/bashrc-{linux,osx}          OS-detected bashrc
 bin/{dt,dtach-router,claude-provider}   CLI scripts deployed to ~/.local/bin
+etc/profile.d/disk-usage-warning.sh     login-time low-disk warning → /etc/profile.d (Linux only)
 .claude/{tasks,memory,audits}/   Claude working state
 ```