feat: add json log format option

Add log_format config field ("text" default, "json" for structured
JSON output) to support machine-readable logging.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -11,7 +11,8 @@ import (
 type Config struct {
 	SSH      SSHConfig  `toml:"ssh"`
 	Auth     AuthConfig `toml:"auth"`
-	LogLevel string     `toml:"log_level"`
+	LogLevel  string `toml:"log_level"`
+	LogFormat string `toml:"log_format"` // "text" (default) or "json"
 }
 
 type SSHConfig struct {
@@ -73,6 +74,9 @@ func applyDefaults(cfg *Config) {
 	if cfg.LogLevel == "" {
 		cfg.LogLevel = "info"
 	}
+	if cfg.LogFormat == "" {
+		cfg.LogFormat = "text"
+	}
 }
 
 func validate(cfg *Config) error {