Add defensive .gitignore patterns for secrets/keys

Blocks .env, *.key, *.pem, id_rsa*, secrets.*, credentials.json, etc.
Preventive — no exposure found; future commits can't accidentally
leak these.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -55,3 +55,22 @@ enrollment/
 drivers-staging/
 bios-staging/
 .claude/
+
+# Secrets and credentials (defensive)
+.env
+.env.*
+!.env.example
+!.env.*.example
+*.pem
+*.key
+id_rsa
+id_rsa.*
+*.ppk
+*.p12
+*.pfx
+secrets.json
+secrets.yaml
+secrets.yml
+*_secret
+*_secrets
+credentials.json