sec(ssh): `Debug` derive on `SshConfig`/`SshTarget`/`BotAuthConfig` leaks credentials in logs

[chaliy]

Summary

`SshConfig`, `SshTarget`, and `BotAuthConfig` all derive `Debug`, which means formatting them with `{:?}` dumps plaintext passwords, private key material, and Ed25519 signing keys. The `SshClient` Debug implementation also includes the full `SshConfig`. Any code path that logs, traces, or includes these structs in error messages will leak credentials.

Threat category: TM-INF (Information Disclosure) — extends TM-INF-016
Severity: Medium
Component:

• `crates/bashkit/src/ssh/config.rs` line 50: `SshConfig` contains `default_password` and `default_private_key`
• `crates/bashkit/src/ssh/handler.rs` line 17: `SshTarget` contains `private_key` and `password`
• `crates/bashkit/src/network/bot_auth.rs` line 31: `BotAuthConfig` contains `signing_key: SigningKey`
• `crates/bashkit/src/ssh/client.rs` line 26: `SshClient` Debug includes full `SshConfig`

Root Cause

```rust
#[derive(Debug, Clone)]
pub struct SshConfig {
pub default_password: Option, // Leaked by Debug
pub default_private_key: Option, // Leaked by Debug
// ...
}

#[derive(Debug, Clone)]
pub struct SshTarget {
pub password: Option, // Leaked by Debug
pub private_key: Option, // Leaked by Debug
// ...
}

#[derive(Debug, Clone)]
pub struct BotAuthConfig {
pub signing_key: SigningKey, // Ed25519 key leaked by Debug
// ...
}
```

Impact

• Password disclosure: SSH passwords appear in any debug log or error message
• Private key leakage: SSH private keys and Ed25519 signing keys exposed
• AI agent context: If these structs are formatted in tool output, credentials flow into LLM context
• Panic backtraces: Panics in code using these types include credential values in the backtrace

Acceptance Criteria

• Implement custom `Debug` for `SshConfig`, `SshTarget`, `BotAuthConfig` that redacts sensitive fields
• Example: `password: Some("[REDACTED]")` instead of the actual value
• Remove `Debug` derive and replace with manual impl for all credential-holding structs
• Audit all `Debug` derives across codebase for other credential-holding types
• Add test: `format!("{:?}", ssh_config)` does not contain the actual password/key