topdata: skip .md doc files in asset validation and packaging (#32)

Docs like AGENTS.md/CLAUDE.md under assets/ broke sow-topdata builds
with "unsupported topdata asset HAK resource extension". Skip .md in
both the validator and HAK package walker.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Reviewed-on: #32
Reviewed-by: xtul <mpiasecki720@protonmail.com>
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.com>

internal/topdata/top_package.go

@@ -214,6 +214,9 @@ func collectTopPackageResources(p *project.Project, compiled2DADir string) ([]er
 			if strings.HasPrefix(filepath.Base(path), ".") {
 				return nil
 			}
+			if strings.EqualFold(filepath.Ext(path), ".md") {
+				return nil // docs (AGENTS.md, README.md, ...) are never HAK resources
+			}
 			resource, err := topPackageResourceFromPath(path)
 			if err != nil {
 				return err