fix: clean up error messages in main

[ogrman]

Summary

• Refactors main() to separate error handling from the core run() function
• Prints errors with lowercase "error:" prefix following standard CLI conventions
• Uses indented "caused by:" for error chains instead of the raw anyhow format
• Includes config file path in error messages for easier debugging

Before:

Error: Reading the config file

Caused by:
    No such file or directory (os error 2)

After:

error: failed to read config file 'nonexistent.yaml'
  caused by: No such file or directory (os error 2)

Test plan

• cargo test passes
• Verified error output with missing config file
• Verified error output with invalid YAML

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