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/__tests__/plugin-bootstrap.test.js

@@ -185,7 +185,6 @@ describe("fast path", () => {
           pluginVersion: pkg.version,
           depsHash: boot.depsHash(),
           pluginRoot: ROOT,
-          skillPluginsSettled: "nothing-missing",
         },
         RT
       );
@@ -206,35 +205,6 @@ describe("fast path", () => {
   });
 });
 
-describe("skill-plugin auto-install gate", () => {
-  // Both cases here must return WITHOUT reaching server/lib/skill-plugins.js —
-  // that module shells out to `claude plugin install`, and no test may.
-  beforeEach(resetRuntime);
-  after(() => delete process.env.AGENTHUB_AUTO_INSTALL_PLUGINS);
-
-  for (const flag of ["0", "false", "off"]) {
-    it(`is off when AGENTHUB_AUTO_INSTALL_PLUGINS=${flag}`, () => {
-      process.env.AGENTHUB_AUTO_INSTALL_PLUGINS = flag;
-      assert.equal(boot.autoInstallSkillPlugins(RT), "off");
-    });
-  }
-
-  it("runs once: a settled state is returned verbatim, not re-attempted", () => {
-    delete process.env.AGENTHUB_AUTO_INSTALL_PLUGINS;
-    boot.writeState({ skillPluginsSettled: "installed" }, RT);
-
-    // Verbatim matters: writeState records whatever comes back, so a marker
-    // here would erase the only record of what actually happened.
-    assert.equal(boot.autoInstallSkillPlugins(RT), "installed");
-  });
-
-  it("is off under the flag even when forced, so /agenthub-update honours the opt-out", () => {
-    process.env.AGENTHUB_AUTO_INSTALL_PLUGINS = "0";
-    boot.writeState({ skillPluginsSettled: "failed" }, RT);
-    assert.equal(boot.autoInstallSkillPlugins(RT, { force: true }), "off");
-  });
-});
-
 describe("bootstrap lock", () => {
   beforeEach(resetRuntime);