Give every Hub execution its own workspace lifecycle (#3045)

* fix(server): archive Hub executions by workspace identity

Hub archive previously depended on backing-specific owner state and archived the agent directly, so local and reused workspaces could survive. Resolve the required workspaceId from the execution agent and let the shared workspace archive service own agent, terminal, and backing teardown.

* fix(server): preserve agent archive through workspace teardown

* test(server): keep Hub regression fixtures workspace-scoped

* fix(hub): own executions through fresh workspaces

Hub executions must own a workspace regardless of backing so archival has one lifecycle boundary for agents, terminals, and checkout cleanup. Retain the retired wire selector for parsing compatibility while removing its ownership semantics.

* fix(server): serialize archive metadata writes

* fix(server): preserve archive during reconciliation

packages/protocol/src/messages.hub.test.ts

@@ -80,6 +80,20 @@ describe("Hub session protocol", () => {
     expect(SessionInboundMessageSchema.parse(message)).toEqual(message);
   });
 
+  test("keeps the retired Hub workspace selector wire-compatible", () => {
+    const message = {
+      type: "hub.execution.agent.create.request",
+      requestId: "request-retired-workspace",
+      executionId: "execution-retired-workspace",
+      provider: "codex",
+      cwd: "/workspace",
+      workspaceId: "caller-owned-workspace",
+      prompt: "Implement the requested change",
+    };
+
+    expect(SessionInboundMessageSchema.parse(message)).toEqual(message);
+  });
+
   test("accepts an optional MCP server configuration on Hub creates", () => {
     const message = {
       type: "hub.execution.agent.create.request",

packages/protocol/src/messages.ts

@@ -2515,6 +2515,7 @@ export const HubExecutionAgentCreateRequestSchema = z.object({
   provider: z.string(),
   cwd: z.string(),
   prompt: z.string(),
+  // COMPAT(hubExecutionWorkspaceSelection): semantics retired in v0.3.1; remove after 2027-08-08 once the Hub floor no longer sends it.
   workspaceId: z.string().optional(),
   model: z.string().optional(),
   modeId: z.string().optional(),