Implement `billing_demo.py` example

[dgenio]

Summary

Demonstrate firewall budgets, handle pagination, and field selection on billing data.

Behavior

1. Set up kernel with billing capabilities + InMemoryDriver
2. Request and invoke "list_invoices" with status="unpaid"
3. Print Frame: facts (count, totals), truncation warning
4. Expand handle: page 2 (offset=50, limit=10) → print results
5. Expand handle: field selection (id, amount, status only) → print results
6. Show "summarize_spend" → print aggregates
7. If capability has PII sensitivity, show redacted fields + warnings

Acceptance criteria

• Runs with python examples/billing_demo.py — exit 0
• Demonstrates budget truncation in output
• Demonstrates pagination and field selection via expand