create a .sketchi folder where generated excalidraw images go by default

[anand-testcompare]

on install or initial generation it should only generate iomages in a gitignored .sketchi folder in the repo root. probably prudent to have sub folders by session id as well

[anand-testcompare]

Folded into #129 scope as an explicit acceptance criterion: move default generated images to gitignored with session-scoped subfolders, plus docs/tests/gitignore updates. Keeping #137 as related tracking context unless you want it closed as duplicate after implementation lands.

[anand-testcompare]

Correction: path is /.sketchi/, with session-scoped subfolders like /.sketchi/sessions/<sessionID>/png/.

[anand-testcompare]

Closing as duplicate scope tracked under #129 (same default-output policy). We can keep execution + acceptance criteria centralized there.

[anand-testcompare]

Correction: duplicate scope is tracked under #129 for the /.sketchi default-output policy and acceptance criteria.