cli: resolve subcommand path in container help (#1587)

- Fixes #1509. The CLI's own help text tells users to
  run `container help <subcommand>`, but every form
  of that results in an error.
- Added a captured subcommand path, walked 
  Application`'s `subcommands` + `groupedSubcommands`
  tree (matching `commandName` and `aliases`), and
  printed `Application.helpMessage(for:)` for the resolved
  target. Empty path keeps existing plugin-aware top-level
  help; unknown path throws `ValidationError`.

Sources/ContainerCommands/HelpCommand.swift

@@ -26,8 +26,43 @@ struct HelpCommand: AsyncLoggableCommand {
     @OptionGroup(visibility: .hidden)
     public var logOptions: Flags.Logging
 
+    @Argument(parsing: .captureForPassthrough)
+    var subcommandPath: [String] = []
+
     func run() async throws {
-        let pluginLoader = try? await Application.createPluginLoader()
-        await Application.printModifiedHelpText(pluginLoader: pluginLoader)
+        if subcommandPath.isEmpty {
+            let pluginLoader = try? await Application.createPluginLoader()
+            await Application.printModifiedHelpText(pluginLoader: pluginLoader)
+            return
+        }
+        guard let target = Self.resolveSubcommand(path: subcommandPath) else {
+            throw ValidationError("unknown command '\(subcommandPath.joined(separator: " "))'")
+        }
+        print(Application.helpMessage(for: target))
+    }
+
+    static func resolveSubcommand(path: [String]) -> ParsableCommand.Type? {
+        var current: ParsableCommand.Type = Application.self
+        for name in path {
+            guard let next = childSubcommands(of: current).first(where: { matches($0, name: name) }) else {
+                return nil
+            }
+            current = next
+        }
+        return current
+    }
+
+    private static func childSubcommands(of command: ParsableCommand.Type) -> [ParsableCommand.Type] {
+        var all = command.configuration.subcommands
+        for group in command.configuration.groupedSubcommands {
+            all.append(contentsOf: group.subcommands)
+        }
+        return all
+    }
+
+    private static func matches(_ command: ParsableCommand.Type, name: String) -> Bool {
+        let cfg = command.configuration
+        if cfg.commandName == name { return true }
+        return cfg.aliases.contains(name)
     }
 }

Tests/ContainerCommandsTests/HelpCommandTests.swift

@@ -0,0 +1,57 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the container project authors.
+//
+// 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 ArgumentParser
+import Foundation
+import Testing
+
+@testable import ContainerCommands
+
+struct HelpCommandTests {
+    @Test
+    func everyStaticSubcommandReachableViaHelp() {
+        func walk(_ command: ParsableCommand.Type, path: [String]) {
+            let cfg = command.configuration
+            var children = cfg.subcommands
+            for group in cfg.groupedSubcommands {
+                children.append(contentsOf: group.subcommands)
+            }
+            for child in children {
+                guard let name = child.configuration.commandName else { continue }
+                let canonical = path + [name]
+                #expect(
+                    HelpCommand.resolveSubcommand(path: canonical) != nil,
+                    "help should resolve '\(canonical.joined(separator: " "))' but returned nil"
+                )
+                for alias in child.configuration.aliases {
+                    let aliasPath = path + [alias]
+                    #expect(
+                        HelpCommand.resolveSubcommand(path: aliasPath) != nil,
+                        "help should resolve alias path '\(aliasPath.joined(separator: " "))' but returned nil"
+                    )
+                }
+                walk(child, path: canonical)
+            }
+        }
+        walk(Application.self, path: [])
+    }
+
+    @Test
+    func unknownSubcommandReturnsNil() {
+        #expect(HelpCommand.resolveSubcommand(path: ["nonexistent"]) == nil)
+        #expect(HelpCommand.resolveSubcommand(path: ["image", "nonexistent"]) == nil)
+    }
+}