feat(config): add production host and infrastructure directory configuration

- Add usage: sunbeam config [-h] action ...

positional arguments:
  action
    set       Set configuration values
    get       Get current configuration
    clear     Clear configuration

options:
  -h, --help  show this help message and exit subcommand with set/get/clear actions
- Store configuration in  with production_host and infra_directory
- Integrate with production environment detection, prioritizing config over SUNBEAM_SSH_HOST
- Add comprehensive test coverage with 11 new tests
- Update CLI help and error messages for better user experience

sunbeam/config.py

@@ -0,0 +1,73 @@
+"""Configuration management — load/save ~/.sunbeam.json for production host and infra directory."""
+import json
+import os
+from pathlib import Path
+from typing import Optional
+
+
+CONFIG_PATH = Path.home() / ".sunbeam.json"
+
+
+class SunbeamConfig:
+    """Sunbeam configuration with production host and infrastructure directory."""
+    
+    def __init__(self, production_host: str = "", infra_directory: str = ""):
+        self.production_host = production_host
+        self.infra_directory = infra_directory
+
+    def to_dict(self) -> dict:
+        """Convert configuration to dictionary for JSON serialization."""
+        return {
+            "production_host": self.production_host,
+            "infra_directory": self.infra_directory,
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> 'SunbeamConfig':
+        """Create configuration from dictionary."""
+        return cls(
+            production_host=data.get("production_host", ""),
+            infra_directory=data.get("infra_directory", ""),
+        )
+
+
+def load_config() -> SunbeamConfig:
+    """Load configuration from ~/.sunbeam.json, return empty config if not found."""
+    if not CONFIG_PATH.exists():
+        return SunbeamConfig()
+    
+    try:
+        with open(CONFIG_PATH, 'r') as f:
+            data = json.load(f)
+        return SunbeamConfig.from_dict(data)
+    except (json.JSONDecodeError, IOError, KeyError) as e:
+        from sunbeam.output import warn
+        warn(f"Failed to load config from {CONFIG_PATH}: {e}")
+        return SunbeamConfig()
+
+
+def save_config(config: SunbeamConfig) -> None:
+    """Save configuration to ~/.sunbeam.json."""
+    try:
+        CONFIG_PATH.parent.mkdir(parents=True, exist_ok=True)
+        with open(CONFIG_PATH, 'w') as f:
+            json.dump(config.to_dict(), f, indent=2)
+        from sunbeam.output import ok
+        ok(f"Configuration saved to {CONFIG_PATH}")
+    except IOError as e:
+        from sunbeam.output import die
+        die(f"Failed to save config to {CONFIG_PATH}: {e}")
+
+
+def get_production_host() -> str:
+    """Get production host from config or SUNBEAM_SSH_HOST environment variable."""
+    config = load_config()
+    if config.production_host:
+        return config.production_host
+    return os.environ.get("SUNBEAM_SSH_HOST", "")
+
+
+def get_infra_directory() -> str:
+    """Get infrastructure directory from config."""
+    config = load_config()
+    return config.infra_directory