chore: gitignore Gas Town agent artifacts (#3633)

Add .beads/, .claude/, and .runtime/ to .gitignore to prevent
agent workspace files from being accidentally committed.

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

Author: mdelapenya

.gitignore

@@ -26,3 +26,8 @@ coverage.out
 
 # Usage metrics script binary
 usage-metrics/scripts/collect-metrics
+
+# Gas Town / Claude Code agent artifacts
+.beads/
+.claude/
+.runtime/