fix(cases): never delete another clone's files after a failed direct clone

A direct clone into an empty folder emptied the folder after any failure, which
deleted a concurrent attempt's finished clone when git had written nothing.
Clones into one folder are now mutually exclusive in-process, the cleanup is
skipped when git refused the folder, never ran, or the mount is unresponsive,
and a name linked while the clone ran is a 409 that keeps the folder.
Documents path for POST /api/cases/clone.

src/web/routes/case-routes.ts

@@ -859,14 +859,16 @@ export function registerCaseRoutes(
         // POST /api/cases/link the write is not serialized, so this only narrows the window.
         try {
           const fresh = await readLinkedCases();
-          if (fresh[name]) throw new Error(`Case "${name}" was linked while the clone ran`);
+          if (fresh[name])
+            throw Object.assign(new Error(`Case "${name}" was linked while the clone ran`), { conflict: true });
           fresh[name] = casePath;
           await fs.writeFile(LINKED_CASES_FILE, JSON.stringify(fresh, null, 2));
         } catch (err) {
           // The clone itself succeeded: keep the folder (the user waited for it) and say where it is.
-          reply.code(500);
+          const conflict = (err as { conflict?: boolean }).conflict;
+          reply.code(conflict ? 409 : 500);
           return createErrorResponse(
-            ApiErrorCode.OPERATION_FAILED,
+            conflict ? ApiErrorCode.ALREADY_EXISTS : ApiErrorCode.OPERATION_FAILED,
             `Cloned into ${casePath} but could not register the case (${getErrorMessage(err)}). Use Link Existing to add it.`
           );
         }

src/web/schemas.ts

@@ -786,7 +786,7 @@ export const CloneCaseSchema = z.object({
   shallow: z.boolean().optional(),
   description: z.string().max(1000).optional(),
   /**
-   * Clone into this exact folder (parent + case name) instead of under the cases directory, and
+   * Clone into this exact folder (the folder itself, no case-name subfolder is added) instead of under the cases directory, and
    * register it as a linked case. Absolute or starting with `~`; only length-bounded here, since
    * `prepareNewCasePath()` (web/case-path.ts) judges it and produces the user-facing reason.
    */