Add PRD for export command

[milldr]

Summary

• New PRD at docs/prds/06-command-export.md for the crono export command
• Uses Cronometer's HTTP export API directly — no Kernel/browser automation needed
• Supports three export types: nutrition, exercises, biometrics
• Plain text default output, with --csv and --json options
• Documents the full auth flow (anticsrf → form login → GWT authenticate → nonce → export)
• GWT magic values hardcoded with config/env overrides for when Cronometer deploys updates
• Migration path to replace Kernel-based diary/weight commands long-term

Test plan

• Review PRD for completeness and accuracy