feat: crash if sessions directory missing after init

[avoidwork]

Description

Move session directory creation from saveSession() to a one-time init function. If the sessions directory disappears after init, saveSession() throws and shutdown handlers treat it as fatal (crash with exit code 1).

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional choice)
• Performance improvement
• CI / build / tooling

Testing

All 955 tests pass including 5 new tests:

• ensureSessionsDir creates directory when missing
• ensureSessionsDir returns successfully when directory already exists
• saveSession writes file successfully when directory exists
• saveSession propagates writeFile errors unhandled
• handleShutdown re-throws errors from saveSession

Coverage

• 100% line coverage maintained for new files (ensureDir.js: 100% line, 100% branch, 100% function)
• Overall coverage maintained at 96.30%

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied

Threat Model Considerations

• Missing sessions directory after init is now a crash condition — this provides a signal that persistence is failing, which is preferred over silent data loss.
• Write failures (disk full, permissions) also crash, providing the OS error message for diagnosis.
• No changes to auth, secrets, or input validation surfaces.