Phase 4-6: Quill editor, Lens viewer, Rigel context menu, CI, website

Phase 4 — native apps:
- quill/: text editor (monospace, double-buffered, find bar, undo, open/save)
- lens/: image viewer using imlib2 (zoom, pan, rotate, directory nav)
- wm/src/menu.c: launch quill instead of mousepad
- iso/docker/entrypoint.sh: add imlib2 package; install quill + lens binaries
- skykit/template/: developer app template demonstrating the SkyKit API

Phase 5 — file system and search:
- rigel/rigel.c: right-click context menu (open, open in quill, copy path,
  rename, trash, properties); Ctrl+F search/filter bar with name highlighting;
  F2 rename-in-place overlay; Delete key moves to trash

Phase 6 — distribution:
- .github/workflows/build-iso.yml: GitHub Actions CI that compiles all
  components and builds a bootable ISO on every tag push; creates a GitHub
  release with sha256 checksums

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

wm/src/menu.c

@@ -8,7 +8,7 @@ static const MenuItem MENU_ITEMS[] = {
     { "App Launcher",      "orionlauncher &"                              },
     { "Terminal",          "alacritty &"                                },
     { "File Manager",      "rigel &"                                    },
-    { "Text Editor",       "mousepad &"                                 },
+    { "Text Editor",       "quill &"                                    },
     { "──────────────────", NULL                                         },
     { "Web Browser",       "firefox &"                                  },
     { "System Monitor",    "alacritty -T 'System Monitor' -e htop &"    },