Filter token scan usage by account

[cbusillo]

Summary

• prevent usage correlation from mixing entries across multiple accounts
• add an explicit --account-id filter for multi-account usage roots
• document the multi-account behavior and add regression coverage

Fixes auto-review finding on session-token-scan usage correlation.

Validation

• python3 -m unittest tools.session-token-scan.test_scan
• ./build-fast.sh