Docs: Remove sessionRetention namespace from settings reference

[afarber]

Summary

Remove incorrectly documented general.sessionRetention from settings reference. This container/namespace setting has showInDialog: false and was never meant to appear in the UI or docs.

Details

The sessionRetention setting is a container object that groups related settings (enabled, maxAge, maxCount, minRetention). Only its child sessionRetention.enabled has showInDialog: true and appears in the /settings UI. The docs had an extra row documenting the parent container as if it were a standalone setting.

Related Issues

Fixes #14623

How to Validate

1. Open docs/cli/settings.md and verify general.sessionRetention row is gone
2. Verify general.sessionRetention.enabled row is still present
3. Run gemini and type /settings - the "Enable Session Cleanup" option should be visible under General (unchanged behaviour)

Pre-Merge Checklist

• [x] Updated relevant documentation and README (if needed)
• [ ] Added/updated tests (if needed)
• [ ] Noted breaking changes (if any)
• [ ] Validated on required platforms/methods:
  • [ ] MacOS
    • [ ] npm run
    • [ ] npx
    • [ ] Docker
    • [ ] Podman
    • [ ] Seatbelt
  • [ ] Windows
    • [ ] npm run
    • [ ] npx
    • [ ] Docker
  • [ ] Linux
    • [ ] npm run
    • [ ] npx
    • [ ] Docker