Remove .env.staging from tracking, gitignore all dstack/.env.* files

Secrets were committed to this public repo in 591c430. All exposed
credentials have been rotated. This commit removes the file from
tracking and prevents future .env files from being committed.

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

Author: amiller

.gitignore

@@ -13,4 +13,5 @@ references/
 .vercel
 .claude/
 dstack/.env
+dstack/.env.*
 dstack/AGENT_*.md