feat: Support SSH config Host alias reference in jump_host configuration

[inureyes]

Summary

Following up on #167, this PR adds support for referencing SSH config Host aliases in bssh's jump_host configuration. This allows users to leverage existing ~/.ssh/config entries with all their settings.

New Formats Supported

Simple string with @ prefix:

clusters:
  internal:
    jump_host: "@bastion"  # References SSH config Host alias

Structured format:

clusters:
  internal:
    jump_host:
      ssh_config_host: bastion  # References SSH config Host alias

How It Works

When an SSH config reference is used, bssh resolves the alias from ~/.ssh/config and extracts:

• HostName (or uses alias as hostname if not specified)
• User
• Port
• IdentityFile (first one, used as SSH key for the jump host)

Example

~/.ssh/config:

Host bastion
    HostName bastion.example.com
    User jumpuser
    Port 2222
    IdentityFile ~/.ssh/bastion_key

bssh config.yaml:

clusters:
  internal:
    nodes:
      - host: internal.private
    jump_host: "@bastion"

This is equivalent to:

    jump_host:
      host: bastion.example.com
      user: jumpuser
      port: 2222
      ssh_key: ~/.ssh/bastion_key

Changes

• Add SshConfigHostRef variant to JumpHostConfig enum
• Add is_ssh_config_ref() and ssh_config_host() methods to JumpHostConfig
• Add resolve_jump_host() and resolve_jump_host_connection() to SshConfig
• Add get_jump_host_with_key_and_ssh_config() for full SSH config resolution
• Update example-config.yaml with SSH config reference examples
• Add 15 new tests for SSH config reference functionality

Test Plan

• New tests for SSH config reference parsing
• New tests for SSH config resolution
• New tests for fallback when alias not found
• Backward compatibility with existing formats
• All 33 jump_host_config tests pass
• Clippy passes with no warnings

Closes #170