Problem

AgentOs.mkdir(path, { recursive: true }) (_mkdirp) probed every ancestor with a read-side exists() before each mkdir:

for (const part of parts) {
  current += `/${part}`;
  if (!(await this.#kernel.exists(current))) {
    await this.#kernel.mkdir(current);
  }
}

On the native sidecar every read-side fs op (exists/stat/readFile) triggers a full shadow-tree reconciliation walk in secure-exec. So a per-component exists() loop makes mkdir -p /a/b/c/d cost O(components × whole-shadow-tree) — a major contributor to the session-creation latency seen on populated VMs (each mkdir step paying a multi-second walk).

Fix

kernel.mkdir already defaults to recursive on both the native sidecar and compat kernels, and creating an existing directory is a no-op, so a single write-side call is sufficient (write-side ops skip the walk):

await this.#kernel.mkdir(path);

Test

Adds a regression test (recursive mkdir issues one mkdir and zero exists() probes) asserting recursive mkdir issues exactly one mkdir and zero exists() probes, while preserving behavior (all intermediate dirs created and writable, idempotent on repeat).

Note: this is the client-side half. The underlying secure-exec bug — read-side fs ops unconditionally re-walking the entire shadow tree — is fixed separately in the secure-exec repo.