chore: replace Docker Desktop references with open-source alternatives

Docker Desktop requires a commercial licence for business use. Replace all
references with free alternatives:
- macOS: Rancher Desktop (GUI) or Colima (CLI)
- Linux: Docker Engine CE (no Desktop needed at all)
- Windows: Rancher Desktop or WSL2 + Docker Engine

setup.sh detects the OS and shows platform-specific install instructions.
claude.sh defers to setup.sh for install hints to avoid duplication.
README documents all options including a WSL2 setup walkthrough.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

claude.sh

@@ -17,18 +17,15 @@ error() { echo -e "${RED}[-]${NC} $*" >&2; }
 # ─── Helpers ──────────────────────────────────────────────────────────────────
 check_deps() {
     if ! command -v docker &>/dev/null; then
-        error "Docker is not installed."
-        error "  → Download Docker Desktop (free): https://www.docker.com/products/docker-desktop/"
+        error "Docker is not installed. Run ./setup.sh for install instructions."
         exit 1
     fi
     if ! docker info &>/dev/null 2>&1; then
-        error "Docker is not running."
-        error "  → Open Docker Desktop and wait for it to finish starting, then try again."
+        error "Docker is not running. Start your Docker runtime, then try again."
         exit 1
     fi
     if ! docker compose version &>/dev/null 2>&1; then
-        error "Docker Compose is not available."
-        error "  → Download Docker Desktop (includes Compose): https://www.docker.com/products/docker-desktop/"
+        error "Docker Compose is not available. Run ./setup.sh for install instructions."
         exit 1
     fi
 }