Fix workspace deletion — use build transition API

The plain DELETE endpoint returns 405 for stopped workspaces. Use
POST /workspaces/:id/builds with {"transition":"delete"} instead,
which is the correct Coder API for workspace deletion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/services/coder.ts

@@ -109,12 +109,14 @@ export class CoderClient {
 
   async deleteWorkspace(workspaceId: string): Promise<void> {
     const res = await fetch(
-      `${this.baseUrl}/api/v2/workspaces/${workspaceId}`,
+      `${this.baseUrl}/api/v2/workspaces/${workspaceId}/builds`,
       {
-        method: "DELETE",
+        method: "POST",
         headers: {
+          "Content-Type": "application/json",
           "Coder-Session-Token": this.token,
         },
+        body: JSON.stringify({ transition: "delete" }),
       },
     );