feat: Implement server CLI interface (bssh-server binary) (#131)

Implement the bssh-server binary with comprehensive CLI interface for
managing the SSH server.

Features:
- Main commands: run, gen-config, hash-password, check-config,
  gen-host-key, version
- Global CLI arguments: -c/--config, -b/--bind-address, -p/--port,
  -k/--host-key, -v/--verbose, -D/--foreground, --pid-file
- Configuration file loading with CLI overrides
- Signal handling for graceful shutdown (SIGTERM, SIGINT)
- Password hashing with bcrypt (cost factor 12)
- SSH host key generation (Ed25519/RSA)
- Configuration validation and checking
- Proper error handling and exit codes

Technical details:
- Uses clap for CLI parsing with derive API
- Integrates with existing ServerFileConfig from issue #130
- Supports both file-based and CLI-based configuration
- Added dependencies: bcrypt 0.16, rand 0.8, ssh-key 0.6
- All clippy checks pass with -D warnings

Resolves #131

Author: inureyes

.gitignore

@@ -25,3 +25,4 @@ Thumbs.db
 .gemini/
 references/
 vendor/
+bssh-server.yaml

Cargo.lock

@@ -54,6 +54,9 @@ tokio-util = "0.7.17"
 shell-words = "1.1.1"
 libc = "0.2"
 ipnetwork = "0.20"
+bcrypt = "0.16"
+rand = "0.8"
+ssh-key = { version = "0.6", features = ["std"] }
 
 [target.'cfg(target_os = "macos")'.dependencies]
 security-framework = "3.5.1"
@@ -72,3 +75,7 @@ mockall = "0.14"
 name = "large_output_benchmark"
 harness = false
 
+[[bin]]
+name = "bssh-server"
+path = "src/bin/bssh_server.rs"
+