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_inpaint.svg

@@ -0,0 +1,12 @@
+<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">
+  <!-- Paint brush -->
+  <path d="M18.37 2.63L14 7l-1.59-1.59a2 2 0 00-2.82 0L8 7l9 9 1.59-1.59a2 2 0 000-2.82L17 10l4.37-4.37a2.12 2.12 0 10-3-3z"/>
+  <path d="M9 8c-2 3-4 3.5-7 4l8 10c2-1 6-5 6-7"/>
+  <path d="M14.5 17.5L4.5 15"/>
+  <!-- AI sparkle -->
+  <circle cx="19" cy="19" r="1" fill="currentColor"/>
+  <path d="M19 16v1"/>
+  <path d="M19 21v1"/>
+  <path d="M16 19h1"/>
+  <path d="M21 19h1"/>
+</svg>