feat(08-06): add dorks list/info/export commands

- Replace cmd/stubs.go dorksCmd stub with full command tree
- Add cmd/dorks.go with list, info, export subcommands
- Wire Registry + custom_dorks merge for list/export
- Bind GITHUB_TOKEN env var via viper for downstream run

Satisfies part of DORK-03 (list/info/export) and DORK-04 (source/category
filtering). run/add/delete land in Task 2.

cmd/stubs.go

@@ -35,11 +35,7 @@ var serveCmd = &cobra.Command{
 	RunE:  notImplemented("serve", "Phase 18"),
 }
 
-var dorksCmd = &cobra.Command{
-	Use:   "dorks",
-	Short: "Manage and run dork queries (Phase 8)",
-	RunE:  notImplemented("dorks", "Phase 8"),
-}
+// dorksCmd is implemented in cmd/dorks.go (Phase 8).
 
 var scheduleCmd = &cobra.Command{
 	Use:   "schedule",