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/lasso.svg

@@ -0,0 +1,6 @@
+<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">
+  <path d="M12 2C6.5 2 2 6.5 2 12c0 3.5 1.8 6.6 4.5 8.4"/>
+  <path d="M12 2c5.5 0 10 4.5 10 10 0 2.5-.9 4.8-2.4 6.5"/>
+  <circle cx="12" cy="20" r="2"/>
+  <path d="M12 18v-6"/>
+</svg>