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>

lib/detect-plugins.sh

@@ -33,7 +33,10 @@ detect_security_guidance() {
 # --- Toggle plugins ---
 
 detect_gstack() {
-  [ -d "$HOME/.claude/skills/gstack" ]
+  # gstack is exposed via per-skill symlinks (browse, canary, qa, …);
+  # the legacy top-level symlink was removed to avoid duplicate entries.
+  # Detect by checking any of its individual skills.
+  [ -L "$HOME/.claude/skills/browse" ] || [ -L "$HOME/.claude/skills/qa" ]
 }
 
 detect_gsd() {