feat: Add distinct exit codes for different error types

[joshrotenberg]

Summary

Add more granular exit codes to help with scripting, pipelines, and CI/CD workflows.

Current Behavior

• 0: Success
• 1: Any error

Proposed Exit Codes

Code	Meaning
0	Success
1	Runtime error (file not found, I/O error, etc.)
2	Expression syntax error (invalid JMESPath)
3	Input parse error (invalid JSON)

Use Cases

• CI pipelines can distinguish "bad query" from "bad input data"
• Shell scripts can handle specific failure modes differently
• Easier debugging when something fails

Notes

• Keep it minimal - 3-4 codes max
• Document in CLI reference
• The "null result" case is intentionally NOT an error (use expressions to check for expected values if needed)