Reject empty session title slugs

[w287346141]

Summary

• keep generated session titles from falling back to the default name when slugging removes all usable text
• return null for punctuation-only generated titles
• add focused title normalization coverage for empty slug cases

Validation

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