Improve error message when config file is missing

[Hell1213]

Issue Description

When the OSS Dev Agent tries to load configuration and the config file is missing, the error message should be more helpful and guide users on how to fix it.

Current Behavior

When is missing, the error message is generic and doesn't tell users:

• What file is missing
• Where it should be located
• How to create it
• What the minimum required configuration is

Expected Behavior

The error message should:

1. Clearly state that the config file is missing
2. Show the expected path (e.g., .ai-agent/config.toml)
3. Provide instructions on how to create it
4. Show an example of minimum required configuration

Files to Modify

• config/loader.py - Add better error message when config file is missing
• Consider adding a helper message or example config

Acceptance Criteria

• Error message clearly states config file is missing
• Error message shows expected file path
• Error message provides instructions to create config
• Error message shows example minimum config
• Error handling is tested

Testing

1. Remove or rename .ai-agent/config.toml
2. Run python3 main.py oss-dev fix <issue_url>
3. Verify the error message is helpful and actionable

Priority

Medium - Improves user experience when setting up the agent