🛡️ Sentinel: [HIGH] Fix MITM vulnerability in ssh2 backend

[adolago]

🚨 Severity: HIGH
💡 Vulnerability: The ssh2 backend (used when ssh2-backend feature is enabled) did not perform host key verification during connection establishment. It called session.handshake() but failed to check the server's host key against known_hosts, allowing any server to be accepted and exposing the connection to Man-in-the-Middle (MITM) attacks.
🎯 Impact: Attackers could intercept SSH connections, potentially stealing credentials or executing malicious commands on the control node or remote hosts if the user enabled the ssh2-backend feature (legacy/alternative to the default russh backend).
🔧 Fix: Implemented verify_host_key in src/connection/ssh.rs. This function loads ~/.ssh/known_hosts (or the configured UserKnownHostsFile), retrieves the server's host key, and verifies it using ssh2::KnownHosts. It respects StrictHostKeyChecking configuration: if strict checking is disabled (default for this tool's config in some contexts, but false in code), it adds new keys to memory (and tries to append to file); if enabled, it rejects unknown hosts. Mismatched keys are always rejected as a security error.
✅ Verification: Verified that the code compiles with cargo check --lib --features ssh2-backend. Confirmed via code inspection that session.host_key() is now checked against known_hosts before authentication proceeds.

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PR created automatically by Jules for task 4155186710690112462 started by @dolagoartur

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