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/__tests__/lanes-api.test.js

@@ -959,6 +959,47 @@ describe("the lane git surface: changes, branches, actions (issue #34)", () => {
     assert.ok(tree.body.entries.some((e) => e.name === "a.txt" && e.dir === false));
   });
 
+  it("saves a file from the editor over the exact bytes it loaded, and nothing else", async () => {
+    const opened = await request("GET", `/api/lanes/${laneId}/git/file?path=a.txt&rev=work`);
+    assert.equal(typeof opened.body.sha, "string");
+
+    const saved = await request("PUT", `/api/lanes/${laneId}/git/file`, {
+      path: "a.txt",
+      content: "one\nTWO\nthree",
+      baseSha: opened.body.sha,
+    });
+    assert.equal(saved.status, 200, JSON.stringify(saved.body));
+    // The final newline the file had survives an editor that drops it.
+    assert.equal(fs.readFileSync(path.join(dir, "a.txt"), "utf8"), "one\nTWO\nthree\n");
+    assert.notEqual(saved.body.sha, opened.body.sha);
+
+    // Saving over the OLD stamp is a conflict, never a silent overwrite.
+    const stale = await request("PUT", `/api/lanes/${laneId}/git/file`, {
+      path: "a.txt",
+      content: "clobbered",
+      baseSha: opened.body.sha,
+    });
+    assert.equal(stale.status, 409);
+    assert.equal(stale.body.error.code, "ESTALE");
+    assert.equal(fs.readFileSync(path.join(dir, "a.txt"), "utf8"), "one\nTWO\nthree\n");
+
+    const escape = await request("PUT", `/api/lanes/${laneId}/git/file`, {
+      path: "../../etc/passwd",
+      content: "x",
+      baseSha: "0".repeat(40),
+    });
+    assert.equal(escape.status, 400);
+    assert.equal(escape.body.error.code, "EBADPATH");
+
+    const secret = await request("PUT", `/api/lanes/${laneId}/git/file`, {
+      path: ".claude/settings.local.json",
+      content: "{}",
+      baseSha: "0".repeat(40),
+    });
+    assert.equal(secret.status, 403);
+    assert.equal(secret.body.error.code, "ESECRET");
+  });
+
   it("stages one file, commits only the index, and refuses an unchanged path", async () => {
     fs.writeFileSync(path.join(dir, "a.txt"), "one\nTHREE\n");
     fs.writeFileSync(path.join(dir, "b.txt"), "left behind\n");