feat: Add SSH jump host support (-J option/ProxyJump)

[inureyes]

Overview

Implement SSH jump host (bastion/proxy) support to enable connections through intermediate SSH servers, supporting the -J option compatible with OpenSSH ProxyJump functionality.

Technical Approach

The implementation leverages russh's open_direct_tcpip_channel capability to create SSH tunnels through jump hosts:

• Connection chaining through nested direct-tcpip channels
• Per-host authentication supporting different methods for each jump
• Connection reuse for multiple operations through same jump chain
• Automatic retry with exponential backoff for resilience
• Full integration with existing authentication and host verification

Implementation Tasks

Phase 1: Core Infrastructure

• Create jump host module (src/jump/mod.rs)
• Implement jump host specification parser for -J format
• Create JumpHostChain struct for connection state management
• Implement single jump host connection establishment
• Add direct-tcpip channel creation through jump host

Phase 2: SSH Client Integration

• Modify SSH client to support channel-based connections
• Route connections through jump chain when -J is specified
• Maintain backward compatibility with direct connections
• Handle authentication for each jump host in chain

Phase 3: Multi-Jump Support

• Implement multiple jump host parsing (comma-separated)
• Create nested channel connections for multi-hop
• Add connection state tracking for each hop
• Implement proper cleanup for connection chains

Phase 4: Error Handling & Recovery

• Add comprehensive error reporting with jump context
• Implement automatic retry with exponential backoff
• Create connection health monitoring
• Add fallback strategies for connection failures

Phase 5: Multi-Host Mode Integration

• Enable jump hosts for cluster operations
• Add YAML configuration support for jump hosts
• Implement connection pooling for jump hosts
• Support different jump hosts per cluster node

Success Criteria

• OpenSSH-compatible -J syntax works identically
• Support 1-5 jump hosts in chain
• Works with all existing bssh commands
• Connection time < 2s for single jump
• Clear error messages showing which jump failed
• Automatic reconnection on transient failures

Examples

# Single jump host
bssh -J jump@bastion user@internal-server

# Multiple jump hosts  
bssh -J jump1@host1,jump2@host2 user@final-destination

# With cluster mode
bssh -C production -J bastion@gateway "command"

Labels

enhancement, help wanted, ssh-compat