🛡️ Sentinel: [HIGH] Fix path traversal in snapshot names

[bitcoiner-dev]

🚨 Severity: HIGH
💡 Vulnerability: The CLI snapshot feature used an unvalidated, user-provided string for the snapshot name to construct the file path using snap_dir.join(format!("{name}.json")). This allows an attacker to inject ../ sequences into the snapshot name to escape the snapshot directory and read or write arbitrary JSON files with the user's permissions.
🎯 Impact: Path Traversal could lead to unauthorized local privilege escalation, overwriting critical data (like the active profile), or accessing files outside the snapshot directory.
🔧 Fix: Introduced crate::utils::validate_file_name to strictly validate that the user-provided snapshot name only contains alphanumeric characters, underscores, and dashes. The function is called before doing any disk operations for save and restore actions.
✅ Verification: Run cargo test --bin zinc-cli to verify no regressions. Code review assessed the validation as robust and complete.

Also appended a security learning to .jules/sentinel.md as per Sentinel's journal process.

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PR created automatically by Jules for task 8522664903168902430 started by @bitcoiner-dev

[google-labs-jules]

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