Refactor: Add core.just and fix universal structure

- Add core.just with common utilities:
  - _detect_runtime: Detect Docker/Podman
  - _detect_compose: Detect compose command
  - env-check: Environment validation
- Fix container.just to be fully universal (remove servass-specific code)
- Move _detect_runtime from images.just to core.just
- Update container.just to use proper universal operations

Universal structure now:
- core.just: Common utilities
- container.just: Universal container operations
- registry.just: Registry authentication
- images.just: Universal image operations

core.just

@@ -0,0 +1,100 @@
+# Core utilities shared across all just-commons modules
+
+# Detect container runtime (Docker or Podman)
+_detect_runtime:
+    #!/usr/bin/env bash
+    if command -v docker >/dev/null 2>&1 && docker info >/dev/null 2>&1; then
+        echo "docker"
+    elif command -v podman >/dev/null 2>&1; then
+        echo "podman"
+    else
+        echo "Error: No container runtime available (docker or podman)" >&2
+        exit 1
+    fi
+
+# Detect compose command based on available and working container runtime
+_detect_compose:
+    #!/usr/bin/env bash
+    # Check Docker first - but make sure daemon is actually running
+    if command -v docker >/dev/null 2>&1 && docker info >/dev/null 2>&1; then
+        # Docker daemon is running, check for compose
+        if docker compose version >/dev/null 2>&1; then
+            echo "docker compose"
+        elif command -v docker-compose >/dev/null 2>&1; then
+            echo "docker-compose"
+        else
+            echo "Error: Docker is running but no compose tool found" >&2
+            exit 1
+        fi
+    # Check Podman if Docker is not available or not running
+    elif command -v podman >/dev/null 2>&1; then
+        if command -v podman-compose >/dev/null 2>&1; then
+            echo "podman-compose"
+        else
+            echo "Error: Podman found but podman-compose not available" >&2
+            exit 1
+        fi
+    else
+        echo "Error: No container runtime (Docker or Podman) found" >&2
+        exit 1
+    fi
+
+# Environment detection and validation
+env-check:
+    #!/usr/bin/env bash
+    set -euo pipefail
+
+    # Colors
+    RED='\033[0;31m'
+    GREEN='\033[0;32m'
+    YELLOW='\033[1;33m'
+    BLUE='\033[0;34m'
+    NC='\033[0m'
+
+    echo "================================================"
+    echo "  Container Environment Detection"
+    echo "================================================"
+    echo ""
+
+    # Check Docker
+    if command -v docker >/dev/null 2>&1; then
+        if docker info >/dev/null 2>&1; then
+            echo -e "${GREEN}✓ Docker Engine detected and running${NC}"
+            docker_available=true
+        else
+            echo -e "${YELLOW}⚠ Docker is installed but daemon is not running${NC}"
+            docker_available=false
+        fi
+    else
+        echo -e "${RED}✗ Docker not found${NC}"
+        docker_available=false
+    fi
+
+    # Check Podman
+    if command -v podman >/dev/null 2>&1; then
+        echo -e "${GREEN}✓ Podman detected${NC}"
+        podman_available=true
+    else
+        echo -e "${RED}✗ Podman not found${NC}"
+        podman_available=false
+    fi
+
+    echo ""
+    echo "================================================"
+    echo "  SUMMARY"
+    echo "================================================"
+
+    if [ "$docker_available" = true ] || [ "$podman_available" = true ]; then
+        echo -e "${GREEN}🎉 Container runtime available!${NC}"
+        echo ""
+        echo "You can use these universal commands:"
+        echo "  just build <project>        # Build any project image"
+        echo "  just start <service>        # Start any service"
+        echo "  just registry-login         # Login to registry"
+    else
+        echo -e "${RED}❌ No container runtime available${NC}"
+        echo ""
+        echo "Please install Docker or Podman:"
+        echo "  • Docker: https://docs.docker.com/get-docker/"
+        echo "  • Podman: https://podman.io/getting-started/installation"
+    fi