fix(ssh): replace derived Debug with manual impls that redact credentials

[chaliy]

Summary

Closes #1189

• Replace #[derive(Debug)] with manual Debug impls for SshConfig and SshTarget that redact default_password, default_private_key, private_key, and password fields with [REDACTED]
• BotAuthConfig was already fixed in fix(network): zeroize bot-auth seed on drop and harden secret handling #1195
• Add tests verifying credentials don't appear in debug output

Why

Formatting these structs with {:?} would dump plaintext passwords and private keys into logs, error messages, panic backtraces, and LLM context.

Test plan

• test_debug_redacts_credentials for SshConfig — verifies password and key are hidden
• test_debug_redacts_credentials for SshTarget — verifies password and key are hidden, host is visible
• All 2319 bashkit unit tests pass