chore: Add .env to gitignore for API keys

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

.gitignore

@@ -1,3 +1,7 @@
+# Environment files (API keys)
+.env
+.env.*
+
 # Context init file (regenerated each session)
 .claude/.context-init.md