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.

server/lib/lanes.js

@@ -11,6 +11,7 @@
 
 const { db } = require("../db");
 const {
+  DEFAULT_PIPELINE_ID,
   listPipelines,
   getPipeline,
   phaseIdx,
@@ -111,7 +112,9 @@ function validateTool(tool) {
  * forever. Reject it at the write, where the caller can still be told.
  */
 function validatePipeline(id) {
-  if (!listPipelines().some((p) => p.id === id)) {
+  // `default` is always valid: with no template installed it is the empty
+  // pipeline every lane sits on (pipelines.js), not a typo.
+  if (id !== DEFAULT_PIPELINE_ID && !listPipelines().some((p) => p.id === id)) {
     throw Object.assign(new Error(`unknown pipeline: ${id}`), { code: "EBADPIPELINE" });
   }
 }