refactor(lanes): drop the workflow skills and Superpowers link; pipelines go dormant

The plugin stops shipping its workflow layer:
- skills/ship-feature and skills/ship-feature-lane, the 18 stage/ship slash
  commands (plugin.json keeps agenthub-doctor/update/open), and all four
  agents/ (qc-local, senior-gate-reviewer, backend-reviewer, frontend-reviewer).
- CLI `agenthub skills install`, `agenthub lanes agents install`,
  `agenthub ship-full|ship-quick`; routes POST /api/skills/install and
  POST /api/lanes/:id/agents/install; the Settings button, the lane Tools
  menu item and the Add-lane setup step that called them.
- The Superpowers auto-install: server/lib/skill-plugins.js, the bootstrap's
  autoInstallSkillPlugins / skillPluginsSettled gate and the doctor row.

Pipelines are kept but dormant: server/data/pipelines/ is deleted, so no
template ships. `default` resolves to EMPTY_PIPELINE (no nodes, progress 0),
createLane still accepts it, POST /api/lanes/:id/stage answers 400
ENOPIPELINE, and the Workspace hides the stage timer, pickers, PipelineMap
and strip stage chip for a lane with no nodes. A template dropped into
DASHBOARD_PIPELINES_DIR turns it all back on. The server suite loads its
templates from server/__tests__/fixtures/pipelines/.

Docs: CLAUDE.md (marker, retired rules), README(.vi), INSTALL, ARCHITECTURE,
docs/LANES|PLUGINS|CLI|API, agenthub-introduction.html.

client/src/pages/Settings.tsx

@@ -699,12 +699,6 @@ export function Settings() {
       return res.ok ? t("hooks.success") : t("hooks.failed");
     });
 
-  const handleSkillsInstall = () =>
-    runAction("skills-install", async () => {
-      const res = await api.settings.skillsInstall();
-      return t("lanesSection.installSkillSuccess", { path: res.path });
-    });
-
   const handleGc = () =>
     runAction("gc", async () => {
       const res = await api.settings.gc({ dryRun: gcDryRun });
@@ -1354,23 +1348,6 @@ export function Settings() {
         <p className="text-xs text-fg-muted mb-4">{t("lanesSection.description")}</p>
 
         <div className="card p-5 space-y-4">
-          <div className="flex items-center justify-between flex-wrap gap-3">
-            <span className="text-xs text-fg-secondary">{t("lanesSection.installSkill")}</span>
-            <button
-              onClick={handleSkillsInstall}
-              disabled={actionLoading !== null}
-              className="btn-ghost text-xs disabled:opacity-50"
-            >
-              {actionLoading === "skills-install" ? (
-                <RefreshCw className="w-3.5 h-3.5 animate-spin" />
-              ) : (
-                <RotateCcw className="w-3.5 h-3.5" />
-              )}
-              {t("lanesSection.installSkill")}
-            </button>
-          </div>
-          {actionBanner(["skills-install"])}
-
           <div className="flex items-center justify-between flex-wrap gap-3">
             <label className="flex items-center gap-2 text-xs text-fg-secondary">
               <input