feat(iso): add slim hook to strip locales/man pages/apt cache from squashfs

Removes ~100-300MB from the squashfs: man pages, non-en locales,
python cache, apt lists and package cache, temp files and logs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/config/hooks/normal/9999-slim.hook.chroot

@@ -0,0 +1,32 @@
+#!/bin/sh
+# 9999-slim.hook.chroot — strip non-essential files to reduce squashfs size.
+set -e
+
+# ── Man pages and documentation ───────────────────────────────────────────────
+find /usr/share/man   -mindepth 1 -delete 2>/dev/null || true
+find /usr/share/doc   -mindepth 1 ! -name 'copyright' -delete 2>/dev/null || true
+find /usr/share/info  -mindepth 1 -delete 2>/dev/null || true
+find /usr/share/groff -mindepth 1 -delete 2>/dev/null || true
+find /usr/share/lintian -mindepth 1 -delete 2>/dev/null || true
+
+# ── Locales — keep only C and en_US ──────────────────────────────────────────
+find /usr/share/locale -mindepth 1 -maxdepth 1 \
+    ! -name 'en' ! -name 'en_US' ! -name 'locale.alias' \
+    -exec rm -rf {} + 2>/dev/null || true
+find /usr/share/i18n/locales -mindepth 1 \
+    ! -name 'en_US' ! -name 'i18n' ! -name 'iso14651_t1' ! -name 'iso14651_t1_common' \
+    -delete 2>/dev/null || true
+
+# ── Python cache ──────────────────────────────────────────────────────────────
+find /usr /opt -name '__pycache__' -type d -exec rm -rf {} + 2>/dev/null || true
+find /usr /opt -name '*.pyc' -delete 2>/dev/null || true
+
+# ── APT cache and lists ───────────────────────────────────────────────────────
+apt-get clean
+rm -rf /var/lib/apt/lists/*
+
+# ── Misc ──────────────────────────────────────────────────────────────────────
+rm -rf /tmp/* /var/tmp/* 2>/dev/null || true
+find /var/log -type f -delete 2>/dev/null || true
+
+echo "=== slim: done ==="