fix: remove .env.example from git tracking to pass secret audit

The audit flags any .env* file in the repository. Removed .env.example
from the git index, added it to .gitignore, and updated README with
inline setup instruction instead.

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

.gitignore

@@ -1,4 +1,5 @@
 .env
+.env.example
 venv/
 __pycache__/
 *.pyc