Add routing integration tests and documentation

- Created routing_integration_test.go with comprehensive routing tests
- Tests cover maintainer/contributor detection, explicit overrides, end-to-end multi-repo
- Added docs/ROUTING.md documenting auto-routing feature
- Closed bd-6u6g, bd-zmi5, bd-nzt4, bd-btsm (routing tests)
- Updated bd-4ms and bd-8hf with progress notes

All routing tests pass. Multi-repo auto-routing is complete.

Amp-Thread-ID: https://ampcode.com/threads/T-2ea8b2ed-ceb7-432e-91f1-1f527b0e7b4d
Co-authored-by: Amp <amp@ampcode.com>

internal/rpc/server_issues_epics.go

@@ -99,6 +99,42 @@ func (s *Server) handleCreate(req *Request) Response {
 	}
 
 	ctx := s.reqCtx(req)
+	
+	// Check if any dependencies are discovered-from type
+	// If so, inherit source_repo from the parent issue
+	var discoveredFromParentID string
+	for _, depSpec := range createArgs.Dependencies {
+		depSpec = strings.TrimSpace(depSpec)
+		if depSpec == "" {
+			continue
+		}
+		
+		var depType types.DependencyType
+		var dependsOnID string
+		
+		if strings.Contains(depSpec, ":") {
+			parts := strings.SplitN(depSpec, ":", 2)
+			if len(parts) == 2 {
+				depType = types.DependencyType(strings.TrimSpace(parts[0]))
+				dependsOnID = strings.TrimSpace(parts[1])
+				
+				if depType == types.DepDiscoveredFrom {
+					discoveredFromParentID = dependsOnID
+					break
+				}
+			}
+		}
+	}
+	
+	// If we found a discovered-from dependency, inherit source_repo from parent
+	if discoveredFromParentID != "" {
+		parentIssue, err := store.GetIssue(ctx, discoveredFromParentID)
+		if err == nil && parentIssue.SourceRepo != "" {
+			issue.SourceRepo = parentIssue.SourceRepo
+		}
+		// If error getting parent or parent has no source_repo, continue with default
+	}
+	
 	if err := store.CreateIssue(ctx, issue, s.reqActor(req)); err != nil {
 		return Response{
 			Success: false,