feat(lanes): split panes on one lane, in-place file editing, hunk review bar

- Split view: every pane shows the selected lane, each in its own tmux
  session (`slot` 0-3 on POST /api/lanes/:id/start; slot 0 stays the
  lane's `run_id`, slots 1-3 are `agenthub-lane-<id>-<slot>`). A pane's
  Active runs hides what the other panes show; stop/reset/remove kill
  every slot. No per-pane lane picker any more.
- Whole-file editor: PUT /api/lanes/:id/git/file (lane-file-write.js)
  with the read path's guards, existing text files only, and a baseSha
  stale check (409 ESTALE). A file it cannot take says why.
- Per-hunk review bar under each hunk's last changed line: prev/next,
  "n of N", Undo, Keep. Review diffs no longer nest their own scroll box
  (unless windowed), so navigation moves the one real scroller.
- Files tree lists untracked-not-ignored files and colours entries by git
  status (M/U/A/D/R), folders holding changes get a dot.
- Changes panel: Unstage all on the single commit/Stage-all row; sizes
  Narrow (30%) / Medium / Full screen, no 70/30.
- Lane card: the needs-you banner is gone (the strip tile keeps it).

docs/API.md

@@ -261,6 +261,8 @@ All fields are optional. The tool (CLI invoked) comes from the lane itself, neve
 - **`approval`** — one of `on-request` or `never`, or omitted/empty for no approval prompt. Passed to `codex -a <value>`.
 - **`resumeSessionId`** or **`resumeLast: true`** — resume an earlier run instead of starting new. For Codex lanes specifically, `resumeLast: true` uses `codex resume --last`; `resumeSessionId` uses `codex resume <sessionId>`. For Claude lanes, `resumeSessionId` re-attaches a Claude session by id.
 
+- **`slot`** — which Workspace split pane's session to start, `0`–`3` (default `0`, else `400 EBADSLOT`). Slot 0 is the lane's own `agenthub-lane-<id>` and is what `run_id` records; slots 1–3 are extra sessions on the same lane, `agenthub-lane-<id>-<slot>`, and never touch `run_id`/`status`. The response adds `run_id`: the session this start opened. `stop`, `reset` and `remove` kill every slot's session, not only `run_id`.
+
 Error `503 ETMUXMISSING` is returned when the host has no `tmux` binary. Every run the dashboard starts is a tmux session, so this is a missing dependency rather than a bad request — the message carries the install command, and the same code comes back from `POST /api/run`. Nothing else in the dashboard is affected.
 
 Error `409 ERUNLIVE` is returned if the lane already has a live run; stop it first with the `stop` action. The same code and status come back when the lane's tmux pane is already running an agent while a `prompt` or `resumeSessionId` was sent — that argv cannot be typed over a live agent, so the request is refused instead of answered 200 with nothing done. A start that asks for neither still adopts the existing pane silently.
@@ -496,6 +498,7 @@ that file.
 ```http
 GET /api/lanes/:id/git/log?limit=30[&file=<path>]
 GET /api/lanes/:id/git/file?path=<path>[&rev=work|HEAD|<sha>]
+PUT /api/lanes/:id/git/file   {path, content, baseSha}
 GET /api/lanes/:id/git/ls?q=<substring>
 GET /api/lanes/:id/git/tree?path=<dir>
 GET /api/lanes/:id/git/tree-all
@@ -510,7 +513,15 @@ and file explorer. All four are read-only and unguarded, and all four answer
   commits touching that path — relative, no `..` segment, else
   `400 EBADPATH`. Reading one commit's diff is
   `/git/changes?range=commit&sha=`.
-- **`/git/file`** → `{available, path, rev, binary, truncated, lines}`.
+- **`PUT /git/file`** saves the whole-file editor (same-origin guarded;
+  `server/lib/lane-file-write.js`). Same path guards as the read, plus: the
+  file must already exist, be text and within the 1 MB cap, and `baseSha`
+  must equal the `sha` it was opened at — otherwise `409 ESTALE`, never an
+  overwrite. `403 ESECRET` for `settings.local.json`; `400 EBADPATH`,
+  `ENOFILE`, `EBADCONTENT`, `ETOOBIG`, `EBINARY`. Answers the file re-read,
+  fresh `sha` included. CRLF endings and the final newline are preserved.
+- **`/git/file`** → `{available, path, rev, binary, truncated, lines, sha?}`
+  (`sha` only for a whole, textual `work` file: what a save sends back).
   `rev=work` (default) is the file on disk — the one branch that reads the
   filesystem, and it is realpath-confined to the lane directory (a symlink
   resolving outside it is `EBADPATH`); anything else goes through
@@ -530,10 +541,12 @@ and file explorer. All four are read-only and unguarded, and all four answer
 - **`/git/ls`** → `{available, paths, truncated, total}`, capped at 500. It
   reads `git ls-files`, so ignored files never appear and no skip-list has to
   be maintained; basename hits rank first.
-- **`/git/tree`** → `{available, path, entries:[{name, path, dir}], truncated}`
-  — one directory level, tracked entries only, directories folded and listed
-  first. `400 EBADPATH` for a path that escapes the lane.
-- **`/git/tree-all`** → `{available, levels: {"<dir>": [{name, path, dir}]}, truncated}` — every level of the tracked tree from ONE `git ls-files`, each level shaped and capped like `/git/tree`'s; the Files tab's **Expand all folders** is this one request instead of one per folder.
+- **`/git/tree`** → `{available, path, entries:[{name, path, dir, status?, changed?}], truncated}`
+  — one directory level: tracked files plus untracked ones git does not
+  ignore (`ls-files --cached --others --exclude-standard`), directories folded
+  and listed first. A changed file carries its porcelain `status` (` M`, `??`,
+  `A `, ` D`…), a folder with anything changed under it `changed: true`. `400 EBADPATH` for a path that escapes the lane.
+- **`/git/tree-all`** → `{available, levels: {"<dir>": [{name, path, dir}]}, truncated}` — every level of the same tree (entries decorated the same way) from ONE `git ls-files`, each level shaped and capped like `/git/tree`'s; the Files tab's **Expand all folders** is this one request instead of one per folder.
 
 #### Read a lane's own branches