Add invoice-generator skill

skills/invoice-generator/SKILL.md

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+---
+name: invoice-generator
+description: Generate and send time tracking invoice Excel files from FreeScout data. Use when the user asks for an invoice, time report, or hours summary for a customer and time period.
+---
+
+# Invoice Generator
+
+Generate Excel invoices from FreeScout time tracking data and send them via WhatsApp.
+
+## Workflow
+
+1. Identify customer name and date range from the user's request
+2. Run: `bin/freescout excel --from YYYY-MM-DD --to YYYY-MM-DD --customer "NAME" --output /tmp/invoice-CUSTOMER-PERIOD.xlsx`
+3. Send the file to the user via the `message` tool
+4. Reply with NO_REPLY (the file message is the reply)
+
+## Available customers
+
+If unsure which customer, run `bin/freescout report --from 2024-01-01 --to 2099-12-31` to see all, or pass a wrong name — the script prints available customers.
+
+## Other commands
+
+```
+bin/freescout mailboxes              # List mailboxes
+bin/freescout users                  # List users  
+bin/freescout report --from --to     # Terminal report (add --customer, --mailbox, --user)
+bin/freescout excel --from --to      # Excel generation (add --customer, --mailbox, --user, --output)
+```
+
+## Notes
+
+- `time_spent` in DB is seconds, displayed as decimal hours
+- Excel groups by conversation: subject, customer, website, total hours
+- Customer and Website come from FreeScout custom fields (dropdowns)
+- Only timelogs within the date range are counted, not conversation creation dates