Disable mimalloc memory scavenger in sandboxed bash subprocesses

When Claude Code's sandbox mode is active, bash subprocesses now have the MIMALLOC_SCAVENGER environment variable set to "0". This disables the mimalloc memory allocator's background scavenger thread, which can cause unexpected memory pressure and instability in constrained sandbox environments.

The change was implemented by extracting inline bash environment construction into a dedicated helper (fUg) that conditionally adds MIMALLOC_SCAVENGER: "0" when shouldUseSandbox is true. The guard function yru() gates the injection on a Bun standalone-executable check: x6i() calls RS(), which returns Bun.isStandaloneExecutable === true. Only when this is true does yru() return { MIMALLOC_SCAVENGER: "0" }; otherwise it returns an empty object {}. This means the variable is injected only when both conditions are met simultaneously — sandbox is active AND Claude Code is running as a Bun standalone binary (the distribution format in which Bun's built-in mimalloc allocator is active). On plain Node.js or other runtimes the spread is a no-op and the subprocess environment is unchanged.