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/ellipse_select.svg

@@ -0,0 +1,7 @@
+<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">
+  <ellipse cx="12" cy="12" rx="10" ry="7" stroke-dasharray="4 2"/>
+  <circle cx="2" cy="12" r="1.5" fill="currentColor"/>
+  <circle cx="22" cy="12" r="1.5" fill="currentColor"/>
+  <circle cx="12" cy="5" r="1.5" fill="currentColor"/>
+  <circle cx="12" cy="19" r="1.5" fill="currentColor"/>
+</svg>