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`.

docs/LANES.md

@@ -420,7 +420,9 @@ The dashboard web UI shows each lane as a card in a grid, with the selected lane
 
 A lane moves through stages defined in a **pipeline template** (see "Custom pipeline templates" below). The default pipeline has eight stages: `intake`, `plan`, `implement`, `tests`, `review`, `gate`, `ship`, and `done`.
 
-**A lane is created on `default` unless told otherwise, and nothing in the web UI changes that** — the "+ Add lane" flow has no template picker, so a lane that should render the 16-node `ship-feature` map will show `default`'s 8 nodes until it is switched:
+**A lane is created on `default` unless told otherwise.** The "+ Add lane" modal has a *Pipeline template* select listing every template the server reports, so this is chosen at creation; `ccam lanes add --pipeline <id>` is the same choice from the terminal. Creation is the point that matters — a lane born on `default` renders 8 nodes for a 16-node workflow.
+
+For a lane that already exists (including every lane created before the picker shipped):
 
 ```bash
 ccam lanes pipeline                 # which template this lane uses, and what else exists