🛡️ Sentinel: [CRITICAL] Fix command injection in slatron-node

[JustinWoodring]

This PR addresses a critical command injection vulnerability in slatron-node's shell_execute Rhai function.

Previously, shell_execute passed the user-provided command string directly to sh -c, allowing arbitrary command execution via shell operators (e.g., ;, |, $()).

The fix:

1. Removes sh -c usage: Commands are now executed directly using std::process::Command, preventing shell interpretation.
2. Implements Strict Allowlist: Only yt-dlp, ffmpeg, ffprobe, and echo are allowed to run.
3. Adds Custom Tokenizer: A helper function tokenize_command splits the command string into executable and arguments while respecting quotes, ensuring arguments are passed correctly without shell expansion.
4. Blocks Dangerous Arguments: Explicitly checks for and blocks --exec in yt-dlp to prevent application-level RCE.
5. Adds Verification Tests: A new test module security_test.rs verifies that injection attempts fail and valid commands work as expected.

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PR created automatically by Jules for task 2193104580598987903 started by @JustinWoodring

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