fix(scripts): add gstack infra symlinks to install, update, and doctor

The gstack/bin/ and gstack/browse/dist/ symlinks (added in link.sh) were
missing from install-plugins.sh and update-all.sh, meaning fresh installs
and updates wouldn't create them. Also adds doctor.sh health checks.

- install-plugins.sh: create symlinks after gstack ./setup
- update-all.sh: refresh symlinks after gstack submodule update
- doctor.sh: verify both symlinks exist in consistency section

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

doctor.sh

@@ -309,6 +309,18 @@ echo ""
 # ────────────────────────────────────────────────────────────
 echo "── Consistency ──"
 
+# Check gstack shared infrastructure symlinks
+if [ -L "$HOME/.claude/skills/gstack/bin" ]; then
+  pass "gstack/bin/ symlink OK"
+else
+  warn "gstack/bin/ symlink missing — run: bash link.sh"
+fi
+if [ -L "$HOME/.claude/skills/gstack/browse/dist" ]; then
+  pass "gstack/browse/dist/ symlink OK"
+else
+  warn "gstack/browse/dist/ symlink missing — run: bash link.sh"
+fi
+
 # Check owned skills have disable-model-invocation (skip external/symlinked skills)
 MISSING_DMI=()
 for f in "$HOME/.claude/skills/"*/SKILL.md; do