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/.gitignore

@@ -0,0 +1,70 @@
+# Python
+__pycache__/
+*.py[cod]
+*$py.class
+*.so
+.Python
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+wheels/
+*.egg-info/
+.installed.cfg
+*.egg
+MANIFEST
+venv/
+env/
+ENV/
+
+# Node
+node_modules/
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+.pnp
+.pnp.js
+coverage/
+build/
+.DS_Store
+.env.local
+.env.development.local
+.env.test.local
+.env.production.local
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# Environment
+.env
+
+# Data
+data/projects/*/
+!data/projects/.gitkeep
+*.db
+*.sqlite
+*.sqlite3
+
+# Docker
+*.log
+docker-compose.override.yml
+
+# Testing
+.pytest_cache/
+.coverage
+htmlcov/
+
+# OS
+.DS_Store
+Thumbs.db