Concurrent Pi startups can race while installing fd or ripgrep

### What happened? When two Pi processes start concurrently for the first time, both can try to install `fd` or `rg` into the shared user-level `agent/bin` directory. They download to the same archive path, and one process can delete the archive while the other is extracting it: ```text Failed to extract fd-10.3.0-aarch64-apple-darwin.tar.gz: tar: ...: No such file or directory ``` One process succeeds while the other returns no tool path, so that session has no file-path completion. ### Steps to reproduce 1. Use a fresh temporary `PI_CODING_AGENT_DIR` shared by two processes. 2. Start two concurrent first-run Pi instances, or call `ensureTool("fd")` concurrently. 3. Delay one download so the other finishes extraction and cleanup first. The second installation fails because both processes use the same archive filename. ### Expected behavior Concurrent first-run installations should not delete or overwrite each other's download and should both return a usable tool path. ### Version Latest `main`