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.

plugins/agenthub/commands/agenthub-doctor.md

@@ -25,12 +25,6 @@ Print the output verbatim, then add at most three lines of guidance based on the
 - `FAIL npm` or `FAIL Disk` → nothing else in the report can be trusted until
   these are fixed; both silently truncate installs.
 - `WARN agenthub CLI` → print the exact `export PATH=...` line from the output.
-- `WARN Skill plugins` → the pipelines name slash commands no installed plugin
-  provides. The dashboard is unaffected; `/agenthub:ship-feature(-lane)` is what
-  stops partway. The bootstrap normally installs these itself, so this row also
-  means that attempt has not run or did not succeed — say `/agenthub-update` retries
-  it, and print the `claude plugin install …` line from the output as the manual
-  route.
 - `WARN systemd` → when it says the unit names a **previous plugin version**,
   the dashboard is running now but will NOT come back after a reboot. Starting
   a new Claude Code session repairs it; print the `--force` command from the