feat: crash if sessions directory missing after init

[avoidwork]

Description

Move directory creation from saveSession() (called on every message) to a single ensureSessionsDir() call at app init. At runtime, if the sessions directory disappears after init, saveSession() validates via stat() and crashes with a fatal error instead of silently creating the directory.

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 changes)
• Performance improvement
• CI / build / tooling

Testing

• OpenSpec change artifacts generated (proposal, design, specs, tasks)
• Pre-commit hooks (oxlint, oxfmt, coverage) passed on commit. Unit tests for ensureSessionsDir, saveSession validation, and handleShutdown error propagation.

Coverage

• 100% line coverage maintained

Checklist

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