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/smart_select.svg

@@ -0,0 +1,15 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <!-- Magic wand with AI sparkle -->
+  <path d="M15 4V2"/>
+  <path d="M15 16v-2"/>
+  <path d="M8 9h2"/>
+  <path d="M20 9h2"/>
+  <path d="M17.8 11.8L19 13"/>
+  <path d="M15 9h0"/>
+  <path d="M17.8 6.2L19 5"/>
+  <path d="M12.2 6.2L11 5"/>
+  <path d="M3 21l9-9"/>
+  <path d="M12.2 11.8L11 13"/>
+  <!-- Selection indicator -->
+  <rect x="2" y="2" width="6" height="6" rx="1" stroke-dasharray="2 1"/>
+</svg>