feat: avoid unnecessary stat on every save

[avoidwork]

Description

saveSession() was calling stat() + mkdir() on every save call. Since every message triggers a save, this means redundant filesystem checks and directory creation fire N times during a session.

This changes saveSession() to a pure writeFile — the directory is ensured once at init via ensureSessionsDir(). If the directory disappears after init, saves fail (crash) instead of silently creating it.

This is an optimization over the existing behavior from #99 — the directory existence guarantee was already in the design, but mkdir was still happening on every save.

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 change)
• Performance improvement
• CI / build / tooling

Testing

Full test suite: 955/955 tests pass. New tests added:

• saver.test.js: saveSession writes file with correct content; defaults filename to unsaved.md
• saver_errors.test.js: saveSession propagates writeFile errors unhandled
• session.test.js: ensureSessionsDir creates directory when missing; returns when directory exists
• shutdown.test.js: handleShutdown re-throws saveSession errors; suppresses telemetry flush errors; calls onShutdown callback; executes saveSession after flushTelemetry

Coverage

• 100% line coverage maintained

Checklist

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