feat(wfctl): add plugin registry install/search/list/update/remove commands

Adds wfctl plugin management subcommands backed by the GoCodeAlone/workflow-registry:

- `plugin search [query]` — lists registry plugins, filters by name/description/keyword
- `plugin install <name>[@ver]` — fetches manifest, downloads OS/arch tarball, verifies
  SHA256 checksum (when present), extracts to data/plugins/<name>/, writes plugin.json
- `plugin list` — scans data/plugins/ and prints installed plugins + versions
- `plugin update <name>` — re-installs latest version from registry
- `plugin remove <name>` — deletes installed plugin directory

New files:
- cmd/wfctl/registry.go: RegistryManifest struct, FetchManifest, ListPluginNames,
  SearchPlugins, FindDownload — registry client using net/http + stdlib only
- cmd/wfctl/plugin_install.go: install flow (gzip+tar extraction with zip-slip guard,
  SHA256 verification), local plugin list/remove helpers
- cmd/wfctl/registry_test.go: unit tests for manifest parsing, OS/arch matching,
  checksum verification, path traversal guard, list/remove commands

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

Author: intel352

cmd/wfctl/plugin.go

@@ -19,6 +19,16 @@ func runPlugin(args []string) error {
 		return runPluginDocs(args[1:])
 	case "test":
 		return runPluginTest(args[1:])
+	case "search":
+		return runPluginSearch(args[1:])
+	case "install":
+		return runPluginInstall(args[1:])
+	case "list":
+		return runPluginList(args[1:])
+	case "update":
+		return runPluginUpdate(args[1:])
+	case "remove":
+		return runPluginRemove(args[1:])
 	default:
 		return pluginUsage()
 	}
@@ -28,9 +38,14 @@ func pluginUsage() error {
 	fmt.Fprintf(flag.CommandLine.Output(), `Usage: wfctl plugin <subcommand> [options]
 
 Subcommands:
-  init   Scaffold a new plugin project
-  docs   Generate documentation for an existing plugin
-  test   Run a plugin through its full lifecycle in a test harness
+  init     Scaffold a new plugin project
+  docs     Generate documentation for an existing plugin
+  test     Run a plugin through its full lifecycle in a test harness
+  search   Search the plugin registry
+  install  Install a plugin from the registry
+  list     List installed plugins
+  update   Update an installed plugin to its latest version
+  remove   Uninstall a plugin
 `)
 	return fmt.Errorf("plugin subcommand is required")
 }