Validate prune session days

[w287346141]

Summary

• require prune-sessions days values to be positive integers
• return immediately after reporting an invalid days value
• add a focused command test for fractional days input

Validation

• npm.cmd test -- --run tests/prune-sessions-command.test.ts tests/session.test.ts
• npm.cmd run lint
• npm.cmd run typecheck
• git diff --check
• push hook ran npm.cmd run verify