Add --log-format option to CLIOptionsRoot

[ephur]

Add a new CLI option to select log output format. Valid values: plain (default), json. This will control behavior in the core logging utility.

[ephur]

🤖 Initial triage (AI-generated):

This is a prerequisite for JSON logging support (see #105). The task is to define a new field in CLIOptionsRoot, likely something like:

log_format: Literal["plain", "json"] = "plain"

⚙️ Suggested enhancements:

• Respect env var override (e.g., LOG_FORMAT)
• Add validation feedback for unsupported values
• Tie into the logging setup logic early in cli()

Disclaimer: This triage is AI-generated and may need refinement.