feat: pin sessions to workspace revisions

backend/test/routes-sessions.test.ts

@@ -603,6 +603,54 @@ test("POST /sessions/:id/resume returns a sanitized error when its retained revi
   expect(response.json()).toMatchObject({ code: "workspace_revision_unavailable" });
 });
 
+test("a pruned pin blocks Resume but not active or mutation lifecycle routes", async () => {
+  const activePath = "/registry/snapshots/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/b-workspace.yaml";
+  const prunedError = "cannot read /registry/snapshots/secret-pruned-revision/b-workspace.yaml";
+  const calls: string[] = [];
+  const readPinned = vi.fn(async () => { throw new Error(prunedError); });
+  let active: any = { bridge: { respond: () => true } };
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    thtRunner: {
+      sessionShow: async (_id: string, workspace: string) => {
+        expect(workspace).toBe(activePath);
+        return {
+          id: "pruned", status: "open", archived: false,
+          workspace_id: "b-workspace", workspace_revision: "b".repeat(40),
+        };
+      },
+      closeSession: async (_id: string, workspace: string) => { calls.push(`close:${workspace}`); },
+      deleteSession: async (_id: string, workspace: string) => { calls.push(`delete:${workspace}`); },
+    } as any,
+    mgr: {
+      get: () => active,
+      teardownIfCurrent: (_id: string, expected: any) => {
+        if (active !== expected) return false;
+        active = undefined;
+        return true;
+      },
+    } as any,
+    workspaceRegistry: {
+      list: async () => [{
+        id: "b-workspace", commit: "a".repeat(40), blob: "a".repeat(40), snapshotPath: activePath, state: "operational",
+      }],
+      readPinned,
+    } as any,
+  });
+
+  expect((await app.inject({ method: "POST", url: "/sessions/pruned/response", payload: { ui_response: {} } })).statusCode)
+    .toBe(204);
+  expect((await app.inject({ method: "POST", url: "/sessions/pruned/close" })).statusCode).toBe(200);
+  expect((await app.inject({ method: "DELETE", url: "/sessions/pruned" })).statusCode).toBe(204);
+  expect(calls).toEqual([`close:${activePath}`, `delete:${activePath}`]);
+  expect(readPinned).not.toHaveBeenCalled();
+
+  const resume = await app.inject({ method: "POST", url: "/sessions/pruned/resume" });
+  expect(resume.statusCode).toBe(409);
+  expect(resume.body).not.toContain(prunedError);
+  expect(resume.json()).toMatchObject({ code: "workspace_revision_unavailable" });
+  expect(readPinned).toHaveBeenCalledWith("b-workspace", "b".repeat(40));
+});
+
 test("POST /sessions/:id/resume refuses a pinned finalized session before reading its snapshot", async () => {
   const readPinned = vi.fn(async () => { throw new Error("must not resolve"); });
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {