chore: gitignore credentials for molecule-ai-workspace-template-langgraph

Adds standard credential gitignore (.env / *.pem / .secrets/ / .auth_token).
Per-CEO directive 2026-04-16: every plugin and template repo should
gitignore credentials so self-hosters can't accidentally commit real
tokens to public repos.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,21 @@
+# Credentials — never commit. Use .env.example as the template.
+.env
+.env.local
+.env.*.local
+.env.*
+!.env.example
+!.env.sample
+
+# Private keys + certs
+*.pem
+*.key
+*.crt
+*.p12
+*.pfx
+
+# Secret directories
+.secrets/
+
+# Workspace auth tokens
+.auth-token
+.auth_token