Rename to stealergram, add pyproject.toml, purge em-dashes

- Rename project to stealergram throughout
- Add pyproject.toml (replaces requirements.txt split, folds pytest.ini)
- Replace all em-dashes with hyphens across all source files

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

utils/cache.py

@@ -1,5 +1,5 @@
 """
-cache.py — Tracks already-processed file IDs to avoid redownloading.
+cache.py - Tracks already-processed file IDs to avoid redownloading.
 Persists to a simple JSON file on disk.
 """