Add comprehensive devcontainer prerequisites validation

[gerchowl]

Description

Improve devcontainer setup experience by adding comprehensive prerequisites validation and documentation.

Changes

Setup Script Enhancement (setup-user-conf.sh)

• Add 5-step validation process:
  1. GHCR authentication (Docker/Podman)
  2. SSH public key validation
  3. Allowed-signers file check
  4. Git configuration validation
  5. GitHub CLI authentication
• Provide detailed setup instructions for each missing prerequisite
• Differentiate between errors (required) and warnings (optional)
• Show actionable commands to fix issues

README Documentation

• Add prominent prerequisite validation tip in Requirements section
• Create comprehensive "Host Machine Prerequisites" section covering:
  • GHCR authentication with token setup
  • Git configuration (with SSH/remote alternatives)
  • SSH key setup with specific naming requirements
  • Allowed-signers file setup
  • GitHub CLI authentication
• Add "Quick Validation - Test All Logins" section
• Update Quick Start with "Test All Logins and Prerequisites" step
• Add clear instructions to run validation script before opening devcontainer

Benefits

• Users can test all logins and authentication before opening devcontainer
• Clear error messages with exact commands to fix issues
• Prevents "login hell" by validating prerequisites upfront
• Better UX for SSH/remote server scenarios