fix(cache): use python for mtime instead of stat (portable across macOS/Linux)

CI failure: scripts/preview-cache.sh used macOS-only stat -f %m. On Linux,
stat -f means "filesystem info" (prints "File: ...") and doesn't fall
through to the -c %Y branch, causing "File: unbound variable" under set -u.

Replaced with python3 os.path.getmtime + time.time() — portable, no platform
branching needed. stdlib-only so still complies with LESSON 0.4 (no deps).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ComBba

scripts/preview-cache.sh

@@ -87,7 +87,8 @@ print(json.load(open('$PLUGIN_ROOT/profiles/$profile_name.json'))['caching']['tt
   fi
 
   if [[ "$ttl" -gt 0 ]]; then
-    local age=$(($(date +%s) - $(stat -f %m "$file" 2>/dev/null || stat -c %Y "$file")))
+    local age
+    age=$(python3 -c "import os,time; print(int(time.time() - os.path.getmtime('$file')))")
     if [[ "$age" -gt "$ttl" ]]; then
       return 1
     fi
@@ -139,7 +140,8 @@ print(json.load(open('$PLUGIN_ROOT/profiles/$profile_name.json'))['caching']['tt
       removed=$((removed + 1))
       continue
     fi
-    local age=$(($(date +%s) - $(stat -f %m "$f" 2>/dev/null || stat -c %Y "$f")))
+    local age
+    age=$(python3 -c "import os,time; print(int(time.time() - os.path.getmtime('$f')))")
     if [[ "$age" -gt "$ttl" ]]; then
       rm -f "$f"
       removed=$((removed + 1))