feat(commands): add SubCommand type and EffectiveUsage method

Introduce SubCommand struct for declaring sub-commands structurally
within a parent command Definition. The EffectiveUsage() method
auto-generates usage strings from sub-command names and args,
preventing drift between help text and actual handler behavior.

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

pkg/commands/definition.go

@@ -1,5 +1,18 @@
 package commands
 
+import (
+	"fmt"
+	"strings"
+)
+
+// SubCommand defines a single sub-command within a parent command.
+type SubCommand struct {
+	Name        string
+	Description string
+	ArgsUsage   string // optional, e.g. "<session-id>"
+	Handler     Handler
+}
+
 // Definition is the single-source metadata and behavior contract for a slash command.
 //
 // Design notes (phase 1):
@@ -10,7 +23,26 @@ package commands
 type Definition struct {
 	Name        string
 	Description string
-	Usage       string
+	Usage       string       // for simple commands; ignored when SubCommands is set
 	Aliases     []string
-	Handler     Handler
+	SubCommands []SubCommand // optional; when set, Executor routes to sub-command handlers
+	Handler     Handler      // for simple commands without sub-commands
+}
+
+// EffectiveUsage returns the usage string. When SubCommands are present,
+// it is auto-generated from sub-command names so metadata and behavior
+// cannot drift.
+func (d Definition) EffectiveUsage() string {
+	if len(d.SubCommands) == 0 {
+		return d.Usage
+	}
+	names := make([]string, 0, len(d.SubCommands))
+	for _, sc := range d.SubCommands {
+		name := sc.Name
+		if sc.ArgsUsage != "" {
+			name += " " + sc.ArgsUsage
+		}
+		names = append(names, name)
+	}
+	return fmt.Sprintf("/%s [%s]", d.Name, strings.Join(names, "|"))
 }

pkg/commands/definition_test.go

@@ -0,0 +1,41 @@
+package commands
+
+import (
+	"testing"
+)
+
+func TestDefinition_EffectiveUsage_NoSubCommands(t *testing.T) {
+	d := Definition{Name: "start", Usage: "/start"}
+	if got := d.EffectiveUsage(); got != "/start" {
+		t.Fatalf("EffectiveUsage()=%q, want %q", got, "/start")
+	}
+}
+
+func TestDefinition_EffectiveUsage_WithSubCommands(t *testing.T) {
+	d := Definition{
+		Name: "show",
+		SubCommands: []SubCommand{
+			{Name: "model"},
+			{Name: "channel"},
+			{Name: "agents"},
+		},
+	}
+	want := "/show [model|channel|agents]"
+	if got := d.EffectiveUsage(); got != want {
+		t.Fatalf("EffectiveUsage()=%q, want %q", got, want)
+	}
+}
+
+func TestDefinition_EffectiveUsage_WithArgsUsage(t *testing.T) {
+	d := Definition{
+		Name: "session",
+		SubCommands: []SubCommand{
+			{Name: "list"},
+			{Name: "resume", ArgsUsage: "<id>"},
+		},
+	}
+	want := "/session [list|resume <id>]"
+	if got := d.EffectiveUsage(); got != want {
+		t.Fatalf("EffectiveUsage()=%q, want %q", got, want)
+	}
+}