fix: refuse manifest sync when project source is remote

Manifest sync would write to manifest.json regardless of the project's
configured manifest source. Projects with ManifestSourceRemote treat
app settings as the source of truth, so writing locally creates
divergence. Refuse early with a clear remediation pointing at
slack app settings.

Author: mwbrooks

internal/pkg/manifest/sync.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 
+	"github.com/slackapi/slack-cli/internal/config"
 	"github.com/slackapi/slack-cli/internal/shared"
 	"github.com/slackapi/slack-cli/internal/shared/types"
 	"github.com/slackapi/slack-cli/internal/slackerror"
@@ -21,6 +22,19 @@ type SyncResult struct {
 // both manifests, computes diffs, prompts the user for resolution, writes
 // the merged result to both the API and the local file, and returns the result.
 func Sync(ctx context.Context, clients *shared.ClientFactory, app types.App, auth types.SlackAuth) (*SyncResult, error) {
+	manifestSource, err := clients.Config.ProjectConfig.GetManifestSource(ctx)
+	if err != nil {
+		return nil, err
+	}
+	if manifestSource.Equals(config.ManifestSourceRemote) {
+		return nil, slackerror.New(slackerror.ErrAppManifestUpdate).
+			WithMessage("Manifest sync is unavailable for projects with remote source of truth").
+			WithRemediation("This project's manifest source is %s. Edit the manifest at %s.",
+				config.ManifestSourceRemote.Human(),
+				style.CommandText("slack app settings"),
+			)
+	}
+
 	localManifest, err := clients.AppClient().Manifest.GetManifestLocal(ctx, clients.SDKConfig, clients.HookExecutor)
 	if err != nil {
 		return nil, slackerror.New("Failed to get local manifest").WithRootCause(err).WithCode(slackerror.ErrInvalidManifest)