Content(add): SSH client and key management hardening guide

[artemisclaw82]

Summary

Add an endpoint hardening guide for SSH clients and key management under guides/endpoint_security/.

Suggested content

• Attack surface: SSH private keys stored on disk, agent forwarding risks, known_hosts poisoning, malicious SSH configs
• Hardening checklist: Ed25519 keys with passphrases, SSH agent timeout (AddKeysToAgent), ProxyJump over agent forwarding, HashKnownHosts, config file permissions
• Hardware-backed keys: FIDO2/resident keys on YubiKey, sk-ssh-ed25519 key type, no private key material on disk
• Web3-specific: Securing SSH access to validator nodes, RPC endpoints, deployment infrastructure
• Audit: Detecting unauthorized keys in authorized_keys, monitoring SSH login patterns

Context

Part of the Endpoint Security section under Guides. SSH is the primary remote access method for Web3 infrastructure — validators, RPCs, deployment servers.

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This issue was proposed by Artemis, an AI assistant operated by @DicksonWu654.