fix: switch aider module to use coder_env instead of inline environment variables

[blink-so]

This PR fixes issue #128 by switching the aider module to use the coder_env resource instead of recommending inline environment variable configuration.

Changes Made

Aider Module (registry/coder/modules/aider/)

• Replaced inline environment variable exports with coder_env resource
• Added coder_env resource to properly set AI provider API keys as environment variables
• Updated README to document the new environment variable handling approach
• Updated tests to reflect the new behavior and verify coder_env resource creation

Key Improvements

1. Reduced cognitive load: Users no longer need to manually configure environment variables on the coder_agent
2. Better separation of concerns: Environment variables are managed by Terraform resources rather than shell scripts
3. Improved maintainability: Centralized environment variable management through the module
4. Follows Terraform best practices: Uses proper resource types for configuration

Technical Details

• The module now creates a coder_env resource when an API key is provided
• Removed all inline export API_KEY= statements from shell commands
• Environment variables are automatically set based on the selected AI provider
• Tests updated to verify the new behavior and ensure no regressions

Testing

• All existing tests pass with the new implementation
• Added verification that coder_env resources are created correctly
• Confirmed that inline environment variable exports are no longer present

This change maintains backward compatibility while providing a cleaner, more maintainable approach to environment variable management in AI agent modules.