fix: rsync double SSH spec in RunRemote, restore dropped flags in Run

- Fix Bug A: RunRemote was re-prefixing destination with user@host: when
  engine.go:135 already pre-prefixed it for remote-to-remote, causing
  "admin@host:admin@host:/path" to be passed to rsync
- Fix Bug B: Run used args[1:] when inserting -e ssh, silently dropping
  the first rsync flag (e.g. -aP became flag-less)
- Fix Bug C: RunRemote extracted source/dest in wrong order for pull
  direction (buildArgs reverses them but RunRemote assumed push order)
- Add rsync_runner_test.go covering buildArgs, RunRemote prefixing,
  and flag preservation

internal/syncengine/rsync_runner.go

@@ -40,16 +40,19 @@ func NewRsyncRunner(sshDir, privKey string) *RsyncRunner {
 }
 
 func (r *RsyncRunner) Run(ctx context.Context, pair *SyncPairConfig, onLine func(stream string, line string)) (*RsyncResult, error) {
-	args := r.buildArgs(pair)
+	cmd := r.buildRsyncCmd(pair)
+	return r.runCmd(ctx, cmd, onLine)
+}
 
-	cmd := exec.CommandContext(ctx, "rsync", args...)
+func (r *RsyncRunner) buildRsyncCmd(pair *SyncPairConfig) *exec.Cmd {
+	args := r.buildArgs(pair)
+	cmd := exec.CommandContext(context.Background(), "rsync", args...)
 	if r.privKey != "" {
 		sshCmd := fmt.Sprintf("ssh -i %s -o StrictHostKeyChecking=accept-new -o UserKnownHostsFile=%s",
 			r.privKey, strings.TrimRight(r.sshDir, "/")+"/known_hosts")
-		cmd.Args = append([]string{"rsync", "-e", sshCmd}, args[1:]...)
+		cmd.Args = append([]string{"rsync", "-e", sshCmd}, args...)
 	}
-
-	return r.runCmd(ctx, cmd, onLine)
+	return cmd
 }
 
 func (r *RsyncRunner) buildArgs(pair *SyncPairConfig) []string {
@@ -93,17 +96,39 @@ func (r *RsyncRunner) RunRemote(ctx context.Context, pair *SyncPairConfig, src *
 		destKey = filepath.Join(r.sshDir, "id_ed25519")
 	}
 
-	destUserHost := fmt.Sprintf("%s@%s", dst.SSHUser, dst.Host)
+	srcUserHost := fmt.Sprintf("%s@%s", src.SSHUser, src.Host)
 	args := r.buildArgs(pair)
 
 	innerSSH := fmt.Sprintf("ssh -i %s -o StrictHostKeyChecking=accept-new -o UserKnownHostsFile=%s",
 		destKey, filepath.Join(r.sshDir, "known_hosts"))
 	rsyncFlags := strings.Join(args[:len(args)-2], " ")
-	sourcePath := args[len(args)-2]
-	destPath := args[len(args)-1]
+
+	var sourcePath, destPath string
+	if pair.Direction == "pull" {
+		sourcePath = args[len(args)-1]
+		destPath = args[len(args)-2]
+	} else {
+		sourcePath = args[len(args)-2]
+		destPath = args[len(args)-1]
+	}
+
+	var remoteSrc, remoteDst string
+	if pair.Direction == "pull" {
+		remoteSrc = sourcePath
+		remoteDst = destPath
+		if !strings.Contains(destPath, "@") {
+			remoteDst = fmt.Sprintf("%s@%s:%s", dst.SSHUser, dst.Host, destPath)
+		}
+	} else {
+		remoteDst = destPath
+		if !strings.Contains(destPath, "@") {
+			remoteDst = fmt.Sprintf("%s@%s:%s", dst.SSHUser, dst.Host, destPath)
+		}
+		remoteSrc = fmt.Sprintf("%s:%s", srcUserHost, sourcePath)
+	}
 
 	remoteCmd := fmt.Sprintf("rsync %s -e %q %s %s",
-		rsyncFlags, innerSSH, sourcePath, destUserHost+":"+destPath)
+		rsyncFlags, innerSSH, remoteSrc, remoteDst)
 
 	sshArgs := []string{
 		"-i", src.PrivKey,