Implement JSON log output support

[ephur]

Enable opt-in JSON-formatted logs using the existing log() utility. Preserve backward compatibility with click.secho() output unless --log-format=json is specified.

[ephur]

🤖 Initial triage (AI-generated):

This enhancement expands the logging layer to support opt-in structured JSON output. Given the centralized log() utility already exists, this will involve:

⚙️ Suggested steps:

• Add log_format as an early CLI option in CLIOptionsRoot
• Branch inside log() based on format setting (plain vs json)
• Maintain click.secho() color output when not in JSON mode
• Support structured fields (see issue Support structured context fields in logging #107)

🧪 Recommend writing integration tests for both formats and ensuring log_level filter behavior is respected.

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