Add SSH client and key management hardening guide

[DicksonWu654]

Summary

• add an endpoint-security guide for SSH client and key management hardening
• cover modern key choices, passphrases, agent forwarding, host verification, automation separation, and revocation
• add the guide to the endpoint-security index and sidebar

Closes #393

Validation

• pnpm exec markdownlint-cli2 docs/pages/guides/endpoint-security/ssh-client-and-key-management-hardening.mdx docs/pages/guides/endpoint-security/index.mdx
• pnpm exec vocs build
• Claude CLI review of the final diff

[github-actions]

Sidebar Configuration Reminder

This PR includes added, renamed, or removed documentation files:

• docs/pages/guides/endpoint-security/ssh-client-and-key-management-hardening.mdx (added)

Please ensure that:

• The sidebar in vocs.config.tsx has been updated to include these files
• New content has the dev: true parameter so it's marked as under development
• Sidebar links match the file paths - use the preview deployment to verify

See Contributing Guide – Sidebar & Navigation for more details.

---

_{This is an automated reminder. If this PR doesn't need sidebar changes, you can ignore this message.}

[github-actions]

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[DicksonWu654]

lgtm