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).

server/routes/lanes.js

@@ -27,6 +27,7 @@ const { broadcast } = require("../websocket");
 const runs = require("../lib/pty-run");
 const { applyRouterToLane } = require("../lib/router-apply");
 const { sameOriginGuard } = require("./run");
+const { writeLaneFile } = require("../lib/lane-file-write");
 const { preflight } = require("../lib/lane-preflight");
 const {
   LANES_ROOT,
@@ -855,6 +856,26 @@ router.get("/:id/git/file", async (req, res) => {
   }
 });
 
+/**
+ * Save one text file in the lane from the viewer's editor. Guards live in
+ * lane-file-write.js (the read path's own, plus existing-textual-file-only and
+ * a `baseSha` stale check). Answers the file re-read, fresh `sha` included.
+ */
+router.put("/:id/git/file", sameOriginGuard, async (req, res) => {
+  const lane = laneOr404(req, res);
+  if (!lane) return;
+  const body = req.body || {};
+  try {
+    await writeLaneFile(lane.cwd, body.path, body.content, body.baseSha);
+    res.json({ available: true, ...(await readFileAt(lane.cwd, body.path, "work")) });
+  } catch (err) {
+    const status =
+      { ESECRET: 403, ESTALE: 409 }[err.code] ??
+      (["EBADPATH", "ENOFILE", "EBADCONTENT", "ETOOBIG", "EBINARY"].includes(err.code) ? 400 : 500);
+    res.status(status).json({ error: { code: err.code || "EWRITE", message: err.message } });
+  }
+});
+
 /** Tracked paths matching `?q=`, for quick-open. */
 router.get("/:id/git/ls", async (req, res) => {
   const lane = laneOr404(req, res);
@@ -877,7 +898,7 @@ router.get("/:id/git/tree-all", async (req, res) => {
   }
 });
 
-/** One directory level of tracked files, for the file explorer. */
+/** One directory level of the explorer tree (tracked + untracked-not-ignored), status-decorated. */
 router.get("/:id/git/tree", async (req, res) => {
   const lane = laneOr404(req, res);
   if (!lane) return;
@@ -1165,26 +1186,44 @@ function wait(ms) {
   return new Promise((resolve) => setTimeout(resolve, ms));
 }
 
-/** Kill a lane run and wait for the tmux session to exit before touching its cwd. */
+/** Every tmux session a lane holds: its recorded `run_id` plus the extra
+ *  split-pane sessions (`agenthub-lane-<id>-<slot>`), which never touch it. */
+function laneRunIds(lane) {
+  const ids = new Set(
+    runs
+      .listRuns()
+      .filter((r) => r.laneId === lane.id)
+      .map((r) => r.id)
+  );
+  if (lane.run_id) ids.add(lane.run_id);
+  return [...ids];
+}
+
+/** Kill a lane's runs and wait for their tmux sessions to exit before touching its cwd. */
 async function stopLaneRun(lane) {
-  if (!lane.run_id) return;
-  try {
-    runs.killRun(lane.run_id);
-  } catch {
-    /* a concurrently completed run is already safe */
+  const ids = laneRunIds(lane);
+  if (ids.length === 0) return;
+  for (const id of ids) {
+    try {
+      runs.killRun(id);
+    } catch {
+      /* a concurrently completed run is already safe */
+    }
   }
 
   const deadline = Date.now() + RUN_EXIT_TIMEOUT_MS;
-  let run = runs.getRun(lane.run_id);
-  while (run && run.status !== "gone") {
-    if (Date.now() >= deadline) {
-      throw lifecycleError(
-        "ERUNTIMEOUT",
-        `lane run ${lane.run_id} did not exit within ${RUN_EXIT_TIMEOUT_MS / 1000} seconds`
-      );
+  for (const id of ids) {
+    let run = runs.getRun(id);
+    while (run && run.status !== "gone") {
+      if (Date.now() >= deadline) {
+        throw lifecycleError(
+          "ERUNTIMEOUT",
+          `lane run ${id} did not exit within ${RUN_EXIT_TIMEOUT_MS / 1000} seconds`
+        );
+      }
+      await wait(RUN_EXIT_POLL_MS);
+      run = runs.getRun(id);
     }
-    await wait(RUN_EXIT_POLL_MS);
-    run = runs.getRun(lane.run_id);
   }
   lanesLib.updateLane(lane.id, { run_id: null });
 }
@@ -1587,9 +1626,18 @@ router.post("/:id/:action", sameOriginGuard, async (req, res) => {
     }
   }
 
+  // The pane a `start` targets: 0 is the lane's own session (`run_id`), 1–3
+  // are the Workspace split panes' extra sessions on the same lane.
+  let startedRunId = null;
   try {
     switch (action) {
       case "start": {
+        const slot = body.slot === undefined ? 0 : body.slot;
+        if (!Number.isInteger(slot) || slot < 0 || slot > runs.MAX_PANE_SLOT) {
+          return res.status(400).json({
+            error: { code: "EBADSLOT", message: `slot must be an integer 0–${runs.MAX_PANE_SLOT}` },
+          });
+        }
         // Overwriting run_id while its child is alive orphans that child: a later
         // reset would kill and await only the RECORDED run, then `git clean -fd`
         // the directory the orphan is still writing into. Stop the first run before starting a
@@ -1603,12 +1651,14 @@ router.post("/:id/:action", sameOriginGuard, async (req, res) => {
           // waited for the lock — the lock makes that race visible instead of
           // spawning a run for a lane that no longer exists.
           if (!current) return { missing: true };
-          const live = current.run_id ? runs.getRun(current.run_id) : null;
+          const liveId = slot ? runs.runIdForLane(current.id, slot) : current.run_id;
+          const live = liveId ? runs.getRun(liveId) : null;
           if (live && live.status === "running") {
             return { conflict: true };
           }
           const handle = runs.spawnRun({
             laneId: current.id,
+            slot,
             initialPrompt: body.prompt || "",
             cwd: current.cwd,
             // The tool is the LANE's fact, never the request's: a client that
@@ -1626,7 +1676,10 @@ router.post("/:id/:action", sameOriginGuard, async (req, res) => {
             resumeLast: body.resumeLast === true,
             resumeSessionId: body.resumeSessionId || (body.resume ? current.session_id : undefined),
           });
-          lanesLib.updateLane(current.id, { run_id: handle.id, status: "running" });
+          startedRunId = handle.id;
+          // An extra pane's session is not the lane's run: `run_id` stays the
+          // pane-0 session every other path (reset, router restart) names.
+          if (!slot) lanesLib.updateLane(current.id, { run_id: handle.id, status: "running" });
           return {};
         });
         if (outcome.missing) {
@@ -1641,9 +1694,9 @@ router.post("/:id/:action", sameOriginGuard, async (req, res) => {
       }
       case "stop": {
         // A lane with no live run is already stopped — say so, don't 500.
-        if (lane.run_id) {
+        for (const id of laneRunIds(lane)) {
           try {
-            runs.killRun(lane.run_id);
+            runs.killRun(id);
           } catch {
             /* already gone */
           }
@@ -1677,7 +1730,10 @@ router.post("/:id/:action", sameOriginGuard, async (req, res) => {
   }
 
   broadcastLane(lane.id);
-  res.json({ lane: payload(lanesLib.getLane(lane.id)) });
+  res.json({
+    lane: payload(lanesLib.getLane(lane.id)),
+    ...(startedRunId ? { run_id: startedRunId } : {}),
+  });
 });
 
 router.delete("/:id", sameOriginGuard, (req, res) => {