Move ai-stack and paintplus vendored source under vendor/

Matches the existing vendor/easy-asterisk convention (used by
services/asterisk.sh) instead of two one-off top-level directories that
cluttered the repo root and didn't look like anything else next to
setup.sh, lib/, services/, extras/. Only the two services' own SRC_DIR
path resolution and header comments needed updating — nothing else in
the repo referenced the old ./ai-stack / ./paintplus paths.

Also documents vendor/ in README.md's Layout section.

vendor/paintplus/frontend/images/icons/ai_edit.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round">
+  <!-- Brush handle -->
+  <path d="M16.5 3.5a2.12 2.12 0 013 3L7 19l-4 1 1-4L16.5 3.5z"/>
+  <!-- AI sparkle star -->
+  <path d="M20 13l.94 2.06L23 16l-2.06.94L20 19l-.94-2.06L17 16l2.06-.94z" fill="currentColor" stroke="none"/>
+  <!-- Small dot -->
+  <circle cx="20" cy="16" r="0" fill="currentColor"/>
+</svg>