fix(gitosync): fill in missing tenant/project from bridge config on reconcile path

When a milestone file's provider identity block omits Tenant and Project
(both are optional since MatchDevelopMilestone fills them from expected),
the bridge's FetchWorkItem call fails with ErrInvalidInput because
planeWorkItemRef requires all three: tenant, project, id.

Fill in missing Tenant/Project from BridgeConfig before building the ref,
mirroring the missing-create path which always uses b.cfg.Tenant/Project.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/core/internal/gitosync/bridge.go

@@ -166,6 +166,12 @@ func (b *Bridge) enqueueDoc(ctx context.Context, scan roadmapsync.ScanResult, do
 	}
 
 	identity := doc.Identity
+	if identity.Tenant == "" {
+		identity.Tenant = b.cfg.Tenant
+	}
+	if identity.Project == "" {
+		identity.Project = b.cfg.Project
+	}
 	ref := workitem.Ref{
 		Provider: identity.Provider,
 		Tenant:   identity.Tenant,