feat(lanes): let Add Lane choose the pipeline template

Creation is the only point the UI could ever set a lane's template, and it
never offered the choice — so every lane added from "+ Add lane" was born
on `default` and rendered an 8-node map for a 16-node workflow, with no
screen able to change it afterwards. That is the defect that made the
ship-feature template unreachable from the browser.

The modal now shows a *Pipeline template* select fed by
`GET /api/lanes/pipelines`, labelled with each template's node count so the
consequence of the choice is visible. A failed fetch degrades to a `default`
option rather than blocking lane creation.

`pipeline` was already accepted by `POST /api/lanes` but silently dropped by
`/ensure` and `/worktree`, which build their own createLane payloads; both
now pass it through, and both map `EBADPIPELINE` to 400 like `EBADCWD`.

client/src/i18n/locales/en/lanes.json

@@ -24,6 +24,8 @@
   "addLaneBranchPlaceholder": "feat/my-feature",
   "addLaneNoBranches": "This repo has no commits yet — the worktree will start empty.",
   "addLaneNotARepo": "Not a git repository (or no read access) yet.",
+  "addLanePipelineHint": "Which stages this lane's map shows. Pick the one the skill driving it declares against — nothing else in this UI changes it later.",
+  "addLanePipelineLabel": "Pipeline template",
   "addLaneProvisionFailed": "Worktree provisioning failed or timed out — the lane exists but auto-setup was skipped. Check the lane's git facts, then run agents/mcp setup manually.",
   "addLaneRepoHint": "An existing git repo. The dashboard creates a new worktree for the lane, not a folder you pick.",
   "addLaneRepoHintAdopt": "An existing directory. The dashboard tracks it as-is — no worktree, no new branch.",