Default sandbox CWD to /app when directory is unset

[phinze]

Follow-up to #621. That PR restored the /app default in the launcher and build server, which fixes new pool creation. But existing pools are persisted in etcd with no directory field in their sandbox spec — the deployment controller happily reuses them, so the launcher default never comes into play.

This adds the /app default in the sandbox controller itself, which is where WithProcessCwd actually gets called. When co.Directory is populated (new pools, apps with custom WORKDIR), it's used as-is. When it's empty (pre-fix pools), we fall back to /app.

Fixes MIR-727

[coderabbitai]

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📝 Walkthrough

Walkthrough

The change modifies the working directory initialization logic for the sandbox container process in controllers/sandbox/sandbox.go. Previously, the process working directory was conditionally applied only when a Directory value was provided. The updated logic now always computes and applies a working directory: defaulting to "/app" when Directory is empty, otherwise using the provided Directory value. This ensures the container process always has a working directory set, removing the previous conditional behavior.

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