feat: structured logging for better readability

[dwmkerr]

Summary

Add structured logging (JSON or logfmt) to make shellwright logs easier to read and filter.

Motivation

Currently logs are plain text. Structured logs would:

• Work with tools like pamburus/hl for colorized, human-readable output
• Enable filtering by log level, session ID, tool name, etc.
• Make debugging easier in production

Proposal

Start simple:

• Add a --log-format flag: text (default, current behavior) or json
• Include useful fields: timestamp, level, session_id, tool, message
• Keep the default human-friendly for basic use

Example JSON output:

{"ts":"2024-12-22T10:30:00Z","level":"info","tool":"shell_start","session":"abc123","msg":"Started bash session"}

Users who want pretty logs can pipe through hl:

npm start 2>&1 | hl