chore: improve .env gitignore pattern and remove .env.example

- Update .gitignore to use .env* wildcard pattern

- Covers all variants: .env.local, .env.backup, .env.production, etc.

- Remove .env.example to prevent accidental secret exposure

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -1,2 +0,0 @@
-ALPHA_VANTAGE_API_KEY=alpha_vantage_api_key_placeholder
-OPENAI_API_KEY=openai_api_key_placeholder

.gitignore

@@ -8,4 +8,4 @@ src/
 eval_results/
 eval_data/
 *.egg-info/
-.env
+.env*