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

internal/linear/types.go

@@ -32,6 +32,7 @@ const (
 type Client struct {
 	APIKey     string
 	TeamID     string
+	ProjectID  string // Optional: filter issues to a specific project
 	Endpoint   string // GraphQL endpoint URL (defaults to DefaultAPIEndpoint)
 	HTTPClient *http.Client
 }