fix(hub): extract PBank zips in-process without host unzip

pbank materialize previously shelled out to `unzip` and soft-failed when
the binary was missing, so agents only saw titles. Read zip entries with
Node zlib (store/deflate) and write under workspace .pbank-sources.

hub/test/unit/pbank-zip.test.ts

@@ -0,0 +1,56 @@
+import { readFile } from "node:fs/promises";
+import { inflateRawSync } from "node:zlib";
+import { describe, expect, it } from "vitest";
+
+/** Mirrors hub/src/capability/pbank.ts zip reader contracts. */
+function listZipEntries(buffer: Buffer) {
+  let eocd = -1;
+  const minEocd = Math.max(0, buffer.length - (22 + 0xffff));
+  for (let i = buffer.length - 22; i >= minEocd; i -= 1) {
+    if (buffer.readUInt32LE(i) === 0x06054b50) {
+      eocd = i;
+      break;
+    }
+  }
+  if (eocd < 0) throw new Error("missing eocd");
+  const totalEntries = buffer.readUInt16LE(eocd + 10);
+  const centralOffset = buffer.readUInt32LE(eocd + 16);
+  const entries: Array<{ name: string; method: number; compressedSize: number; uncompressedSize: number; localHeaderOffset: number }> = [];
+  let offset = centralOffset;
+  for (let i = 0; i < totalEntries; i += 1) {
+    const method = buffer.readUInt16LE(offset + 10);
+    const compressedSize = buffer.readUInt32LE(offset + 20);
+    const uncompressedSize = buffer.readUInt32LE(offset + 24);
+    const nameLen = buffer.readUInt16LE(offset + 28);
+    const extraLen = buffer.readUInt16LE(offset + 30);
+    const commentLen = buffer.readUInt16LE(offset + 32);
+    const localHeaderOffset = buffer.readUInt32LE(offset + 42);
+    const nameStart = offset + 46;
+    const name = buffer.subarray(nameStart, nameStart + nameLen).toString("utf8");
+    entries.push({ name, method, compressedSize, uncompressedSize, localHeaderOffset });
+    offset = nameStart + nameLen + extraLen + commentLen;
+  }
+  return entries;
+}
+
+function inflateZipEntry(buffer: Buffer, entry: { method: number; compressedSize: number; uncompressedSize: number; localHeaderOffset: number }) {
+  const local = entry.localHeaderOffset;
+  const nameLen = buffer.readUInt16LE(local + 26);
+  const extraLen = buffer.readUInt16LE(local + 28);
+  const dataStart = local + 30 + nameLen + extraLen;
+  const compressed = buffer.subarray(dataStart, dataStart + entry.compressedSize);
+  if (entry.method === 0) return Buffer.from(compressed);
+  if (entry.method === 8) return Buffer.from(inflateRawSync(compressed));
+  throw new Error(`method ${entry.method}`);
+}
+
+describe("pbank zip reader contract", () => {
+  it("lists and inflates deflated entries without host unzip", async () => {
+    const zip = await readFile(new URL("./fixtures/pbank-mini.zip", import.meta.url));
+    const entries = listZipEntries(zip);
+    expect(entries.map((e) => e.name).sort()).toEqual(["fig/a.png", "hello.txt"]);
+    const hello = entries.find((e) => e.name === "hello.txt")!;
+    const text = inflateZipEntry(zip, hello).toString("utf8");
+    expect(text).toBe("hello pbank\n");
+  });
+});