chore: bootstrap gstack team mode

Per gstack team-mode install: adds a PreToolUse hook that blocks
skill usage when gstack isn't installed globally, so contributors
are prompted to install it. Un-ignores the two required files
(.claude/settings.json, .claude/hooks/check-gstack.sh) while
keeping settings.local.json and other Claude state ignored.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -207,7 +207,11 @@ marimo/_lsp/
 __marimo__/
 
 # Claude Code (local config, agents, settings)
-.claude/
+.claude/*
+!.claude/settings.json
+!.claude/hooks/
+.claude/hooks/*
+!.claude/hooks/check-gstack.sh
 .agents/
 
 # Database dumps