feat: Implement SSH client with authentication support (Task 5)

- Add SSH client with Connect/Execute/Close/IsConnected
- Implement password, key, and both authentication methods
- Support default key discovery (~/.ssh/id_ed25519, id_rsa, etc.)
- Support passphrase-protected keys via ParsePrivateKeyWithPassphrase
- Path expansion for ~ in KeyID
- Integrate with HandleSSHError for user-friendly errors
- All 4 SSH tests passing + 5 error tests still passing (9 total)

Progress: Tasks 1-5 complete (~40%)

test/ssh/ssh_test.go

@@ -0,0 +1,106 @@
+package ssh_test
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"git.tukangketik.id/swanadiva/hostkeeper/internal/models"
+	"git.tukangketik.id/swanadiva/hostkeeper/pkg/ssh"
+)
+
+func TestNewClient(t *testing.T) {
+	host := &models.Host{
+		ID:       "test-host",
+		Name:     "Test Server",
+		Hostname: "localhost",
+		Port:     22,
+		Username: "testuser",
+		Auth: models.AuthConfig{
+			Type:     "password",
+			Password: "testpass",
+		},
+	}
+
+	client := ssh.NewClient(host, 30*time.Second)
+	if client == nil {
+		t.Fatal("Failed to create SSH client")
+	}
+
+	if client.IsConnected() {
+		t.Error("Expected client to not be connected initially")
+	}
+
+	// Close should be safe even when not connected
+	if err := client.Close(); err != nil {
+		t.Errorf("Expected nil error on close when not connected, got %v", err)
+	}
+}
+
+func TestConnectFailure(t *testing.T) {
+	// Test connecting to a non-existent server
+	host := &models.Host{
+		ID:       "test-host",
+		Name:     "Non-existent Server",
+		Hostname: "127.0.0.1",
+		Port:     9999, // Port that's likely not running SSH
+		Username: "testuser",
+		Auth: models.AuthConfig{
+			Type:     "password",
+			Password: "testpass",
+		},
+	}
+
+	client := ssh.NewClient(host, 2*time.Second)
+	ctx := context.Background()
+	err := client.Connect(ctx)
+
+	// We expect connection to fail
+	if err == nil {
+		t.Log("Connection succeeded (unexpected - SSH server may be running on port 9999)")
+		_ = client.Close()
+	} else {
+		t.Logf("Connection failed as expected: %v", err)
+	}
+}
+
+func TestExecuteWithoutConnection(t *testing.T) {
+	host := &models.Host{
+		ID:       "test-host",
+		Name:     "Test Server",
+		Hostname: "localhost",
+		Port:     22,
+		Username: "testuser",
+		Auth: models.AuthConfig{
+			Type:     "password",
+			Password: "testpass",
+		},
+	}
+
+	client := ssh.NewClient(host, 30*time.Second)
+	ctx := context.Background()
+
+	_, err := client.Execute(ctx, "echo hello")
+	if err == nil {
+		t.Error("Expected error when executing command without connection")
+	}
+}
+
+func TestGetClient(t *testing.T) {
+	host := &models.Host{
+		ID:       "test-host",
+		Name:     "Test Server",
+		Hostname: "localhost",
+		Port:     22,
+		Username: "testuser",
+		Auth: models.AuthConfig{
+			Type:     "password",
+			Password: "testpass",
+		},
+	}
+
+	client := ssh.NewClient(host, 30*time.Second)
+	if client.GetClient() != nil {
+		t.Error("Expected nil underlying client before connecting")
+	}
+}