feat(sagi): spawn a real claude session for Sagi's Ask handoff

Unmatched Ask questions now adopt a dedicated "Sagi" lane (POST
/api/lanes/ensure, cwd defaults to $HOME) and spawn a tmux-backed claude
run there instead of just prefilling the Run page composer.

server/routes/lanes.js

@@ -161,11 +161,25 @@ router.get("/branches", async (req, res) => {
  */
 router.post("/ensure", sameOriginGuard, (req, res) => {
   const body = req.body || {};
-  const owner = lanesLib.resolveLaneByCwd(body.cwd);
-  if (owner) return res.json({ lane: payload(owner), created: false });
+  // Default the cwd to the user's home so callers that don't care about a
+  // specific directory (e.g. Sagi's mascot companion) get a real, existing
+  // path and lane uniqueness stays meaningful. Callers that do care pass one
+  // through unchanged.
+  const cwd = typeof body.cwd === "string" && body.cwd.trim() ? body.cwd.trim() : os.homedir();
+  const owner = lanesLib.resolveLaneByCwd(cwd);
+  if (owner) {
+    // First-write wins on the title: an ensure-with-title on an existing lane
+    // is how callers like Sagi adopt the lane, so don't clobber a real title
+    // with an empty one.
+    if (body.title && !owner.title) {
+      const updated = lanesLib.updateLane(owner.id, { title: body.title });
+      return res.json({ lane: payload(updated || owner), created: false });
+    }
+    return res.json({ lane: payload(owner), created: false });
+  }
   try {
     const lane = lanesLib.createLane({
-      cwd: body.cwd,
+      cwd,
       title: body.title || "",
       ...(body.pipeline ? { pipeline: body.pipeline } : {}),
     });
@@ -178,7 +192,7 @@ router.post("/ensure", sameOriginGuard, (req, res) => {
     // The cwd UNIQUE constraint is the arbiter: someone else won the race, so
     // re-read and return THEIR lane rather than reporting a conflict.
     if (err.code === "SQLITE_CONSTRAINT_UNIQUE" || String(err.message).includes("UNIQUE")) {
-      const winner = lanesLib.resolveLaneByCwd(body.cwd);
+      const winner = lanesLib.resolveLaneByCwd(cwd);
       if (winner) return res.json({ lane: payload(winner), created: false });
     }
     return res.status(500).json({ error: { code: err.code, message: err.message } });