feat(superpowers): vendor the 7 wired skills at v6.4.1, drop the plugin

plugins.lock.json gains a superpowers entry (obra/superpowers @ 5bf4e78,
path skills, per-skill file lists, always_on) that lib/vendor-skills.sh
fetches byte-for-byte: brainstorming, writing-plans,
subagent-driven-development, test-driven-development,
requesting-code-review, using-git-worktrees, writing-skills. install-plugins
STEP 8e vendors it, update-all refreshes it at the pin, link.sh links the
seven, .gitignore ignores them. The plugin is no longer installed or
protected: its 8 other skills duplicated personal flows and its
SessionStart injection cost ~900 tokens per start, clear and compact.
detect_superpowers is one file test on the linked skill; doctor and
session-start stop charging the injection. doctor-vendored gains an
always_on class (third lock column) so always-on externals are
link-checked instead of reported parked.

link.sh

@@ -79,7 +79,9 @@ EXTERNAL_SKILLS=(emil-design-eng frontend-design design-motion-principles
   observability-and-instrumentation deprecation-and-migration ci-cd-and-automation
   scroll-world-storytelling build-threejs-scroll-worlds
   scroll-scrubbed-visual-sequence scroll-scrubbed-word-reveal
-  scroll-progress-timeline)
+  scroll-progress-timeline brainstorming writing-plans
+  subagent-driven-development test-driven-development
+  requesting-code-review using-git-worktrees writing-skills)
 for _ext_skill in "${EXTERNAL_SKILLS[@]}"; do
   if [ -d "$REPO/skills-external/$_ext_skill" ]; then
     if [ -L "$CLAUDE/skills/$_ext_skill" ] && [ "$(readlink "$CLAUDE/skills/$_ext_skill")" = "$REPO/skills-external/$_ext_skill" ]; then