fix: allow clearing unavailable worktree choice (#114947)

ui/src/e2e/new-session-page.workspace-validation.e2e.test.ts

@@ -142,7 +142,7 @@ suite.define(() => {
     }
   });
 
-  it("blocks a custom worktree when Git rediscovery is unavailable", async () => {
+  it("allows clearing a custom worktree when Git rediscovery is unavailable", async () => {
     const context = await suite.browser.newContext({ locale: "en-US", serviceWorkers: "block" });
     const page = await context.newPage();
     const gateway = await installMockGateway(page, {
@@ -155,6 +155,7 @@ suite.define(() => {
           defaultBranch: "main",
           repositoryStatus: "git",
         },
+        "sessions.create": { key: "agent:main:custom-worktree-cleared" },
       },
     });
 
@@ -192,11 +193,23 @@ suite.define(() => {
       await expect.poll(() => start.isDisabled()).toBe(true);
       await trigger.click();
       const worktree = place.getByRole("button", { name: "Worktree" });
-      expect(await worktree.isDisabled()).toBe(true);
+      expect(await worktree.isEnabled()).toBe(true);
       expect(await worktree.getAttribute("title")).toBe(
         "Couldn't verify Git for this folder. Choose it again to retry.",
       );
-      expect(await gateway.getRequests("sessions.create")).toHaveLength(0);
+      await worktree.click();
+      await expect.poll(() => trigger.getAttribute("data-worktree")).toBe("false");
+      await expect.poll(() => start.isEnabled()).toBe(true);
+      await page.keyboard.press("Escape");
+
+      await start.click();
+      const create = await gateway.waitForRequest("sessions.create");
+      expect(create.params).toMatchObject({
+        agentId: "main",
+        cwd: TARGET_REPO,
+        message: "do not run directly",
+      });
+      expect(create.params).not.toHaveProperty("worktree");
     } finally {
       await context.close();
     }

ui/src/pages/new-session/place-picker.ts

@@ -399,7 +399,11 @@ export function renderPlaceSelect(params: {
                         value: "worktree",
                         label: t("newSession.worktree"),
                         checked: params.worktree,
-                        disabled: Boolean(params.cloudProfileId) || !params.worktreeAvailable,
+                        // Failed discovery blocks enabling Worktree, but an existing selection
+                        // must stay actionable so the user can clear the submit-blocking state.
+                        disabled:
+                          Boolean(params.cloudProfileId) ||
+                          (!params.worktreeAvailable && !params.worktree),
                         title: params.cloudProfileId
                           ? t("newSession.cloudRequiresWorktree")
                           : params.worktreeAvailable