feat(lanes): full-width Changes, Stage all, one-read expand-all

- The Changes panel's width control gains a full-width step (narrow →
  50/50 → 70/30 → full → closed); the console beside it is hidden with
  CSS, never unmounted.
- "Expand all folders" is one GET /api/lanes/:id/git/tree-all (one
  `git ls-files`, every level shaped like /git/tree) instead of one
  request and one git process per folder.
- Overview: a Stage all button and the staged-only commit box sit above
  the file list and stay pinned while it scrolls; the Files finder is
  pinned too.

docs/LANES.md

@@ -575,7 +575,7 @@ says more than any sentence the dashboard could write.
 
 To the right of the lane detail sits a collapsible **Changes** card (its state
 persists per viewer in `localStorage`). Its header holds **one** control, which
-steps narrow → 50/50 → 70/30 → closed (`agenthub.workspace.changesSize`
+steps narrow → 50/50 → 70/30 → full width (the console beside it is hidden, never unmounted) → closed (`agenthub.workspace.changesSize`
 alongside `agenthub.workspace.changesOpen`); the collapsed rail reopens it
 narrow. Reading a diff and watching a console want opposite amounts of room and
 which one is "work" changes several times an hour, so the width is a step, not
@@ -638,7 +638,9 @@ in every panel narrow enough to sit beside a console.
 
 **Per-file writes are the panel's, per-repo writes are the git bar's.** Staging
 one file means nothing unless a commit can take the index as it stands, so the
-overview grows a commit box the moment anything is staged — it sends
+overview grows a commit box the moment anything is staged — above the list and
+pinned while it scrolls, beside a **Stage all** that stages every listed file
+the index lacks in one call — it sends
 `only: "staged"`, never `add -A`, and it is absent with a clean index because
 there is nothing it could mean. It is the dashboard's ONLY commit: the git bar
 above it has none, so no click can sweep the rest of the tree in with `add -A`. **Discard** is the one