🔒 fix command injection in sandbox proxy command execution

[TrueAlpha-spiral]

The vulnerability was located in packages/cli/src/utils/sandbox.ts where spawn was called with shell: true using a command string from the GEMINI_SANDBOX_PROXY_COMMAND environment variable. This allowed for command injection.

The fix involves:

1. Parsing the proxyCommand string into an array of arguments using the parse function from the shell-quote library.
2. Filtering the parsed arguments to ensure only strings are passed (removing any shell operators like |, &&, etc. that shell-quote might identify).
3. Invoking spawn with the parsed command and arguments, and removing the shell: true option.

A reproduction security test was added in packages/cli/src/utils/sandbox_repro.security.test.ts to verify the fix.

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PR created automatically by Jules for task 16733833787291557428 started by @TrueAlpha-spiral

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