Add Banqline CLI usage skill

[flemzord]

Summary

• add a repo-local Codex skill for operating and troubleshooting the Banqline CLI
• document the core auth-vs-sync mental model to avoid confusing valid sessions with populated cache
• include detailed workflows for setup, sync, accounts, balances, transactions, reports, forecasts, tags, alerts, multi-bank usage, output formats, and troubleshooting

Validation

• quick_validate.py .codex/skills/banqline-cli-usage
• git diff --cached --check
• checked generated skill files are ASCII and contain no TODO/template markers

Notes

• This is documentation/skill content only; no Rust code changed.