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/lib/api.ts

@@ -1130,8 +1130,6 @@ export const api = {
         purged_events: number;
         purged_agents: number;
       }>("/settings/cleanup", { method: "POST", body: JSON.stringify(params) }),
-    skillsInstall: () =>
-      request<{ installed: true; path: string }>("/skills/install", { method: "POST", body: "{}" }),
     gc: (body: { dryRun?: boolean } = {}) =>
       request<{ reaped: number[]; capped: { path: string; sizeBefore: number }[] }>("/lanes/gc", {
         method: "POST",
@@ -2719,11 +2717,6 @@ export const api = {
           body: JSON.stringify(body),
         }),
     },
-    agentsInstall: (id: number) =>
-      request<{ installed: string[] }>(`/lanes/${id}/agents/install`, {
-        method: "POST",
-        body: "{}",
-      }),
     /**
      * POST /lanes/:id/router/apply - point this lane at an AI Router profile,
      * or back at `native`. Writes the lane's own config on disk; only the