Add firewall logging and fix direnv .env loading

[richardkiene]

Summary

This PR introduces two key improvements to the devcontainer:

• Optional firewall logging: Adds configurable, rate-limited logging for debugging network connectivity issues
• Automatic .env loading: Fixes direnv configuration to automatically load environment variables from .env files

Features Added

Firewall Logging

• Rate-limited logging (10 packets/minute by default) to prevent log spam
• Configurable via environment variables (FIREWALL_LOG_ENABLED, FIREWALL_LOG_RATE)
• Helps debug blocked network requests during development
• Comprehensive documentation in docs/firewall-logging.md
• Logging configuration added to docker-compose.yml and .env.example

Direnv Environment Loading

• Configures direnv with dotenv_if_exists to automatically load .env files
• Adds .envrc template file for immediate use
• Updates documentation with environment variable loading instructions
• Improves developer experience by eliminating manual environment setup

Test Plan

• Verify firewall logging works when enabled via environment variables
• Test that .env files are automatically loaded when entering directories
• Confirm rate limiting prevents log spam
• Validate that existing functionality remains unaffected
• Test direnv integration in both bash and zsh shells

Both features enhance the development experience while maintaining the container's security model.