feat: Implement bssh-keygen tool

[inureyes]

Summary

• Implement bssh-keygen tool for SSH key pair generation in OpenSSH format
• Support Ed25519 (default, recommended) and RSA algorithms
• Include comprehensive tests and CLI interface

Features

• Ed25519 key generation (default, recommended)
• RSA key generation with configurable key size (2048-16384 bits)
• OpenSSH private key format output
• OpenSSH public key format (.pub file)
• Proper file permissions (0600 for private key on Unix)
• Comment support (-C flag)
• SHA256 fingerprint display
• Overwrite confirmation (-y to skip)
• Quiet mode (-q)
• Generated keys are fully compatible with OpenSSH

New Files

File	Description
src/keygen/mod.rs	Module exports and public API
src/keygen/ed25519.rs	Ed25519 key generation
src/keygen/rsa.rs	RSA key generation
src/bin/bssh_keygen.rs	CLI binary

Usage

# Generate Ed25519 key (default)
bssh-keygen

# Generate with custom output path
bssh-keygen -f ~/.ssh/my_key

# Generate RSA key with 4096 bits
bssh-keygen -t rsa -b 4096

# Generate with custom comment
bssh-keygen -C "user@hostname"

# Generate without confirmation prompt
bssh-keygen -y

CLI Help

Generate SSH key pairs in OpenSSH format

Usage: bssh-keygen [OPTIONS]

Options:
  -t, --type <TYPE>        Key type: ed25519 (recommended) or rsa [default: ed25519]
  -f, --file <FILE>        Output file path (default: ~/.ssh/id_<type>)
  -b, --bits <BITS>        RSA key bits (only for RSA, minimum 2048) [default: 4096]
  -C, --comment <COMMENT>  Comment for the key
  -y, --yes                Overwrite existing files without prompting
  -q, --quiet              Quiet mode (no output except errors)
  -v, --verbose...         Verbosity level (-v, -vv, -vvv)
  -h, --help               Print help
  -V, --version            Print version

Test Plan

• Unit tests for Ed25519 key generation
• Unit tests for RSA key generation (various sizes)
• Unit tests for key file permissions (0600)
• Unit tests for public key format
• CLI parsing tests
• Integration test: Generated keys work with ssh-keygen -y
• Integration test: Fingerprint matches ssh-keygen output

Closes #143