feat(toggle): enable/disable non-marketplace tools via lib/toggle-external.sh

`claude plugin enable|disable` only toggles marketplace plugins.
Tools living as symlinks (gstack per-skill entries, emil-design-eng,
darwin-skill, find-skills) had no lever — users had to edit symlinks
by hand. The new script moves symlinks in/out of skills-disabled/
so Claude Code stops or starts scanning them.

Also removes the legacy global `skills/gstack` symlink that shadowed
per-skill entries with a duplicate top-level "gstack" skill (same
description as "browse"). gstack detection in detect-plugins.sh now
probes an individual skill instead.

plugin-advisor reads the new script's `list` command when gathering
state and emits its `enable|disable` commands in recommendations.

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

.gitignore

@@ -48,6 +48,9 @@ skills/emil-design-eng
 skills/darwin-skill
 skills/find-skills
 
+# Staging area used by lib/toggle-external.sh when disabling a tool
+skills-disabled/
+
 # Local project config (per-machine, not shared)
 .claude/