fix(cli): create shared cache volumes from devcontainer initializeCommand

[adamw]

Problem

On a host where `sandcat run` has never been used, VS Code's "Reopen in Container" fails:

```
external volume "sandcat-cache-gradle" not found
```

The generated `compose-all.yml` declares the `sandcat-cache-*` volumes as external. Only the `sandcat run` wrapper created them. The IDE calls compose directly and skips that step.

Fix

• New host-side script `.devcontainer/sandcat/scripts/ensure-cache-volumes.sh`. It reads the `sandcat-cache-*` volume names from `compose-all.yml` with grep, and runs `docker volume create` for each with the same label `sandcat run` uses. Needs only bash, grep, awk and docker on the host.
• `devcontainer.json` template runs it as `initializeCommand`, which executes on the host before compose.
• Docs note in the shared caches page.

Verification

• bats: new tests for the script and the template entry, whole `init` suite passes.
• Ran the script against real Docker: volumes get created with the label, second run is a no-op.
• Not verified inside VS Code or JetBrains Gateway themselves. Both document `initializeCommand` as supported.