feat(linear): add project_id filter for sync (#938)

Add project_id filter for Linear sync

When linear.project_id is configured, bd linear sync will only fetch issues
belonging to that project instead of all team issues.

Closes #937

cmd/bd/linear.go

@@ -26,6 +26,7 @@ var linearCmd = &cobra.Command{
 Configuration:
   bd config set linear.api_key "YOUR_API_KEY"
   bd config set linear.team_id "TEAM_ID"
+  bd config set linear.project_id "PROJECT_ID"  # Optional: sync only this project
 
 Environment variables (alternative to config):
   LINEAR_API_KEY - Linear API key
@@ -586,6 +587,10 @@ func getLinearClient(ctx context.Context) (*linear.Client, error) {
 		if endpoint, _ := store.GetConfig(ctx, "linear.api_endpoint"); endpoint != "" {
 			client = client.WithEndpoint(endpoint)
 		}
+		// Filter to specific project if configured
+		if projectID, _ := store.GetConfig(ctx, "linear.project_id"); projectID != "" {
+			client = client.WithProjectID(projectID)
+		}
 	}
 
 	return client, nil