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/pencil.svg

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="iso-8859-1"?>
+<!-- Generator: Adobe Illustrator 19.0.0, SVG Export Plug-In . SVG Version: 6.00 Build 0)  -->
+<svg version="1.1" id="Capa_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
+	 viewBox="0 0 512 512" style="enable-background:new 0 0 512 512;" xml:space="preserve">
+<g>
+	<g>
+		<polygon points="51.2,353.28 0,512 158.72,460.8 		"/>
+	</g>
+</g>
+<g>
+	<g>
+		
+			<rect x="89.73" y="169.097" transform="matrix(0.7071 -0.7071 0.7071 0.7071 -95.8575 260.3719)" width="353.277" height="153.599"/>
+	</g>
+</g>
+<g>
+	<g>
+		<path d="M504.32,79.36L432.64,7.68c-10.24-10.24-25.6-10.24-35.84,0l-23.04,23.04l107.52,107.52l23.04-23.04
+			C514.56,104.96,514.56,89.6,504.32,79.36z"/>
+	</g>
+</g>
+</svg>