Make "Plugin not found" message more descriptive (#569)

Users frequently saw “failed to find plugin …” when the system services
weren’t running; the message didn’t explain the root cause or where
plugins are looked up. No change to plugin execution flow; only error
messaging and path discovery hints are improved for a better UX.

Sources/CLI/DefaultCommand.swift

@@ -18,6 +18,7 @@ import ArgumentParser
 import ContainerClient
 import ContainerPlugin
 import Darwin
+import Foundation
 
 struct DefaultCommand: AsyncParsableCommand {
     static let configuration = CommandConfiguration(
@@ -46,8 +47,50 @@ struct DefaultCommand: AsyncParsableCommand {
             throw ValidationError("Unknown option '\(command)'")
         }
 
+        // Compute canonical plugin directories to show in helpful errors (avoid hard-coded paths)
+        let installRoot = CommandLine.executablePathUrl
+            .deletingLastPathComponent()
+            .appendingPathComponent("..")
+            .standardized
+        let userPluginsURL = PluginLoader.userPluginsDir(installRoot: installRoot)
+        let installRootPluginsURL =
+            installRoot
+            .appendingPathComponent("libexec")
+            .appendingPathComponent("container")
+            .appendingPathComponent("plugins")
+            .standardized
+        let hintPaths = [userPluginsURL, installRootPluginsURL]
+            .map { $0.appendingPathComponent(command).path(percentEncoded: false) }
+            .joined(separator: "\n  - ")
+
+        // If plugin loader couldn't be created, the system/APIServer likely isn't running.
+        if pluginLoader == nil {
+            throw ValidationError(
+                """
+                Plugins are unavailable. Start the container system services and retry:
+
+                    container system start
+
+                Check to see that the plugin exists under:
+                  - \(hintPaths)
+
+                """
+            )
+        }
+
         guard let plugin = pluginLoader?.findPlugin(name: command), plugin.config.isCLI else {
-            throw ValidationError("failed to find plugin named container-\(command)")
+            throw ValidationError(
+                """
+                Plugin 'container-\(command)' not found.
+
+                - If system services are not running, start them with: container system start
+                - If the plugin isn't installed, ensure it exists under:
+
+                Check to see that the plugin exists under:
+                  - \(hintPaths)
+
+                """
+            )
         }
         // Before execing into the plugin, restore default SIGINT/SIGTERM so the plugin can manage signals.
         Self.resetSignalsForPluginExec()

Tests/CLITests/Subcommands/Plugins/TestCLIPluginErrors.swift

@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025 Apple Inc. and the container project authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import Testing
+
+struct TestCLIPluginErrors {
+    @Test
+    func testHelpfulMessageWhenPluginsUnavailable() throws {
+        // Intentionally invoke an unknown plugin command. In CI this should run
+        // without the APIServer started, so DefaultCommand will fail to create
+        // a PluginLoader and emit the improved guidance.
+        let cli = try CLITest()
+        let (_, stderr, status) = try cli.run(arguments: ["nosuchplugin"])  // non-existent plugin name
+
+        #expect(status != 0)
+        #expect(stderr.contains("container system start"))
+        #expect(stderr.contains("Plugins are unavailable") || stderr.contains("Plugin 'container-"))
+        // Should include at least one computed plugin search path hint
+        #expect(stderr.contains("container-plugins") || stderr.contains("container/plugins"))
+    }
+}