fix(server-mode): a session-only share resumes in a free slot only

A start that names no slot means pane 0; for a user with only a session of
the workspace that was let through and answered ERUNLIVE whenever another
user's agent held pane 0 (found by the browser run). The client already
sends slot: "auto"; the scope now requires it.

server/__tests__/lane-shares.test.js

@@ -479,6 +479,7 @@ describe("sharing one session", () => {
     });
     await refused("POST", "/api/run", { laneId: laneS, initialPrompt: "hi" });
     await refused("POST", `/api/lanes/${laneS}/start`, { resumeSessionId: "s1-shared", slot: 0 });
+    await refused("POST", `/api/lanes/${laneS}/start`, { resumeSessionId: "s1-shared" });
     await refused("GET", `/api/lanes/${laneS}/git/tree?source=1`);
     await refused("GET", `/api/artifacts/candidates?laneId=${laneS}`);
     await refused("PATCH", "/api/sessions/s1-shared", { name: "mine now" });
@@ -494,7 +495,10 @@ describe("sharing one session", () => {
     db.prepare("INSERT INTO session_shares (session_id, user_id) VALUES ('sT-other', ?)").run(
       users.carol.id
     );
-    await refused("POST", `/api/lanes/${laneT}/start`, { resumeSessionId: "s1-shared" });
+    await refused("POST", `/api/lanes/${laneT}/start`, {
+      resumeSessionId: "s1-shared",
+      slot: "auto",
+    });
     db.prepare("DELETE FROM session_shares WHERE session_id = 'sT-other'").run();
     // Another session of it is not even there.
     const other = await call("POST", `/api/lanes/${laneS}/start`, {
@@ -505,7 +509,7 @@ describe("sharing one session", () => {
     // A resume of the shared one gets past the scope (and then needs their own account).
     const resume = await call("POST", `/api/lanes/${laneS}/start`, {
       as: "carol",
-      body: { resumeSessionId: "s1-shared", prompt: "hi" },
+      body: { resumeSessionId: "s1-shared", prompt: "hi", slot: "auto" },
     });
     assert.ok(![403, 404].includes(resume.status), JSON.stringify(resume.body));
     // Reads of the workspace's git and its own row pass.

server/lib/scope.js

@@ -399,8 +399,9 @@ function sessionShareAllows(req, rest, laneId) {
       b.forkSession !== true &&
       b.shell !== true &&
       b.resume !== true &&
-      // A free slot the server picks: pane 0 is the workspace's own run.
-      (b.slot === undefined || b.slot === "auto") &&
+      // A free slot the server picks (no slot means pane 0, the workspace's
+      // own run — another user's live agent there would answer ERUNLIVE).
+      b.slot === "auto" &&
       sessionSharedWith(b.resumeSessionId, memberId(req)) &&
       Boolean(
         sql("SELECT 1 FROM sessions WHERE id = ? AND lane_id = ?").get(b.resumeSessionId, laneId)