fix(ci): remove accidental .claude/skills submodule gitlink

Commit 015fd5e ("fix: add charts CSS to base layout") inadvertently
committed .claude/skills as a submodule (mode 160000) without adding
a .gitmodules entry. Every subsequent actions/checkout invocation on
main has failed its submodule cleanup with:

    fatal: No url found for submodule path '.claude/skills' in .gitmodules

This caused OpenSSF Scorecard runs to fail outright and Gitleaks /
Trivy to emit checkout warnings on every push and on the weekly
scheduled run.

Drop the gitlink from the index and ignore .claude/skills/ and
.claude/worktrees/ so future agent state doesn't slip back into the
tree. .claude/commands/ and .claude/settings.local.json.example
remain tracked as before.

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

Author: sarg3nt

.claude/skills

@@ -16,6 +16,11 @@ gearbox-agent/.env
 *.local.json
 .mcp.json
 
+# Claude Code per-developer state (commands/ and settings.local.json.example
+# are intentionally tracked; everything else is local-only)
+.claude/skills/
+.claude/worktrees/
+
 # Development
 .DS_Store
 *.swp