🛡️ Sentinel: [CRITICAL] Fix arbitrary command execution via unvalidated binary path

[bitcoiner-dev]

🚨 Severity: CRITICAL
💡 Vulnerability: The CLI application allowed users to supply an arbitrary executable path for bitcoin_cli in their profile configuration. This path was passed directly to std::process::Command::new without any validation. While Command::new uses execve directly and mitigates classic shell injection in the arguments, it does not prevent an attacker or a malicious configuration from substituting the intended binary with an arbitrary system binary (like /bin/sh or a trojan executable) to gain complete control under the context of the running application.
🎯 Impact: Arbitrary command execution leading to complete host takeover, data theft (including sensitive wallet information), and privilege escalation.
🔧 Fix: Implemented validate_bitcoin_cli_path in src/utils.rs which performs strict validation on the given path. It asserts that the filename component is exactly bitcoin-cli or bitcoin-cli.exe, and explicitly blocks directory traversal attempts (..). This validation is now applied before initializing std::process::Command::new in both invocations of run_bitcoin_cli across the codebase.
✅ Verification: Run cargo test --bin zinc-cli to ensure there are no regressions. Attempt to run the CLI with a malicious bitcoin_cli path in the profile (e.g. /bin/sh) to verify that the CLI correctly rejects it with a configuration error instead of executing it.

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PR created automatically by Jules for task 11265635129825612101 started by @bitcoiner-dev

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