fix(update): pass full name@marketplace to claude plugin update

`claude plugin update` rejects a bare plugin name when multiple
marketplaces are registered — the CLI demands `name@marketplace`.
update-all.sh stripped the suffix via `${_p%%@*}`, causing every
marketplace plugin update to fail with "Plugin not found". Fixed
by passing the unmodified spec from `claude plugin list`.

Also adds install + update paths for external skills distributed
through the `npx skills` CLI (vercel-labs/skills):
  - alchaincyf/darwin-skill
  - alchaincyf/find-skills

These land in ~/.agents/skills/ and are now symlinked into
$REPO/skills/ via link.sh using absolute paths — the previous
relative `../../.agents/...` targets resolved incorrectly when
the repo is cloned below $HOME (as ~/Documents/claude/), leaving
dangling symlinks.

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

.gitignore

@@ -44,6 +44,10 @@ skills/unfreeze
 # Emil Design Engineering skill symlink — auto-created by link.sh
 skills/emil-design-eng
 
+# External skills installed via `npx skills add` — auto-created by link.sh
+skills/darwin-skill
+skills/find-skills
+
 # Local project config (per-machine, not shared)
 .claude/