Suggest and install official extensions for unknown commands

When a user runs an unknown command that matches a known official
extension (e.g. `gh stack` or `gh aw`), show an install suggestion.
In interactive TTY sessions, prompt the user to install it on the spot.

The official extension registry is hard-coded in pkg/extensions/.
Current entries are github/gh-aw and github/gh-stack. Teams can add
their extensions via PR. Install suggestions use an explicit github.com
host prefix for GHES compatibility.

Refs github/cli#220

Co-Authored-By: Copilot <223556219+Copilot@users.noreply.github.com>

internal/ghcmd/cmd.go

@@ -14,15 +14,18 @@ import (
 
 	surveyCore "github.com/AlecAivazis/survey/v2/core"
 	"github.com/AlecAivazis/survey/v2/terminal"
+	"github.com/MakeNowJust/heredoc"
 	"github.com/cli/cli/v2/api"
 	"github.com/cli/cli/v2/internal/agents"
 	"github.com/cli/cli/v2/internal/build"
 	"github.com/cli/cli/v2/internal/config"
 	"github.com/cli/cli/v2/internal/config/migration"
+	"github.com/cli/cli/v2/internal/prompter"
 	"github.com/cli/cli/v2/internal/update"
 	"github.com/cli/cli/v2/pkg/cmd/factory"
 	"github.com/cli/cli/v2/pkg/cmd/root"
 	"github.com/cli/cli/v2/pkg/cmdutil"
+	"github.com/cli/cli/v2/pkg/extensions"
 	"github.com/cli/cli/v2/pkg/iostreams"
 	"github.com/cli/cli/v2/utils"
 	"github.com/cli/safeexec"
@@ -140,6 +143,18 @@ func Main() exitCode {
 			return exitCode(extError.ExitCode())
 		}
 
+		// Check if any of the provided args match a known official extension.
+		// We scan all args rather than just the first because global flags
+		// (e.g. --repo) may precede the unknown command name.
+		if strings.HasPrefix(err.Error(), "unknown command ") {
+			for _, arg := range expandedArgs {
+				if ext := extensions.FindOfficialExtension(arg); ext != nil {
+					handleOfficialExtension(cmdFactory.IOStreams, cmdFactory.Prompter, cmdFactory.ExtensionManager, ext, err)
+					return exitError
+				}
+			}
+		}
+
 		printError(stderr, err, cmd, hasDebug)
 
 		if strings.Contains(err.Error(), "Incorrect function") {
@@ -245,3 +260,41 @@ func isUnderHomebrew(ghBinary string) bool {
 	brewBinPrefix := filepath.Join(strings.TrimSpace(string(brewPrefixBytes)), "bin") + string(filepath.Separator)
 	return strings.HasPrefix(ghBinary, brewBinPrefix)
 }
+
+// handleOfficialExtension prints a suggestion for the matched official extension
+// and, in interactive TTY sessions, prompts the user to install it.
+func handleOfficialExtension(io *iostreams.IOStreams, p prompter.Prompter, em extensions.ExtensionManager, ext *extensions.OfficialExtension, err error) {
+	stderr := io.ErrOut
+
+	fmt.Fprintln(stderr, err)
+
+	if !io.CanPrompt() {
+		fmt.Fprint(stderr, heredoc.Docf(`
+			%q is also available as an official extension.
+			To install it, run:
+			  gh extension install github.com/%s/%s
+		`, fmt.Sprintf("gh %s", ext.Name), ext.Owner, ext.Repo))
+		return
+	}
+
+	prompt := heredoc.Docf(`
+		%q is also available as an official extension.
+		Would you like to install it now?
+	`, fmt.Sprintf("gh %s", ext.Name))
+	confirmed, promptErr := p.Confirm(prompt, true)
+	if promptErr != nil || !confirmed {
+		return
+	}
+
+	repo := ext.Repository()
+	io.StartProgressIndicatorWithLabel(fmt.Sprintf("Installing %s/%s...", ext.Owner, ext.Repo))
+	defer io.StopProgressIndicator()
+	installErr := em.Install(repo, "")
+	io.StopProgressIndicator()
+	if installErr != nil {
+		fmt.Fprintf(stderr, "Failed to install extension: %s\n", installErr)
+		return
+	}
+
+	fmt.Fprintf(stderr, "Successfully installed %s/%s\n", ext.Owner, ext.Repo)
+}

pkg/extensions/official.go

@@ -0,0 +1,40 @@
+package extensions
+
+import (
+	"github.com/cli/cli/v2/internal/ghrepo"
+)
+
+// OfficialExtension describes a GitHub-owned CLI extension that can be
+// suggested to users when they invoke an unknown command.
+type OfficialExtension struct {
+	Name  string
+	Owner string
+	Repo  string
+}
+
+// Repository returns a ghrepo.Interface pinned to github.com for use with
+// ExtensionManager.Install.
+func (e *OfficialExtension) Repository() ghrepo.Interface {
+	return ghrepo.NewWithHost(e.Owner, e.Repo, "github.com")
+}
+
+// officialExtensions is the hard-coded registry of GitHub-owned extensions
+// that gh will suggest installing when the user invokes an unknown command
+// matching one of their names.
+// Install suggestions include the "github.com/" host prefix so that GHES users
+// install from github.com rather than their enterprise host.
+var officialExtensions = []OfficialExtension{
+	{Name: "aw", Owner: "github", Repo: "gh-aw"},
+	{Name: "stack", Owner: "github", Repo: "gh-stack"},
+}
+
+// FindOfficialExtension returns the matching official extension for
+// commandName, or nil if none matches.
+func FindOfficialExtension(commandName string) *OfficialExtension {
+	for _, ext := range officialExtensions {
+		if ext.Name == commandName {
+			return &ext
+		}
+	}
+	return nil
+}

pkg/extensions/official_test.go

@@ -0,0 +1,41 @@
+package extensions
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFindOfficialExtension(t *testing.T) {
+	tests := []struct {
+		name        string
+		commandName string
+		wantNil     bool
+		wantRepo    string
+	}{
+		{name: "found", commandName: "stack", wantNil: false, wantRepo: "gh-stack"},
+		{name: "not found", commandName: "xyzzy", wantNil: true},
+		{name: "empty", commandName: "", wantNil: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ext := FindOfficialExtension(tt.commandName)
+			if tt.wantNil {
+				assert.Nil(t, ext)
+			} else {
+				require.NotNil(t, ext)
+				assert.Equal(t, tt.wantRepo, ext.Repo)
+			}
+		})
+	}
+}
+
+func TestOfficialExtension_Repository(t *testing.T) {
+	ext := &OfficialExtension{Name: "stack", Owner: "github", Repo: "gh-stack"}
+	repo := ext.Repository()
+	assert.Equal(t, "github", repo.RepoOwner())
+	assert.Equal(t, "gh-stack", repo.RepoName())
+	assert.Equal(t, "github.com", repo.RepoHost())
+}