feat(command): extend slash command system with new commands and UX improvements

Add 9 new command groups (/model, /memory, /search, /browser, /usage,
/email, /heartbeat, /skill, /fs) and improve existing commands by hiding
internal UUIDs, resolving IDs to human-readable names in /settings, and
switching /schedule to name-based references.

internal/command/registry.go

@@ -0,0 +1,92 @@
+package command
+
+import (
+	"context"
+	"fmt"
+	"strings"
+)
+
+// CommandContext carries execution context for a sub-command.
+type CommandContext struct {
+	Ctx   context.Context
+	BotID string
+	Role  string // "owner", "admin", "member", or "" (guest)
+	Args  []string
+}
+
+// SubCommand describes a single sub-command within a resource group.
+type SubCommand struct {
+	Name    string
+	Usage   string
+	IsWrite bool
+	Handler func(cc CommandContext) (string, error)
+}
+
+// CommandGroup groups sub-commands under a resource name.
+type CommandGroup struct {
+	Name          string
+	Description   string
+	DefaultAction string
+	commands      map[string]SubCommand
+	order         []string // preserves registration order for help output
+}
+
+func newCommandGroup(name, description string) *CommandGroup {
+	return &CommandGroup{
+		Name:        name,
+		Description: description,
+		commands:    make(map[string]SubCommand),
+	}
+}
+
+func (g *CommandGroup) Register(sub SubCommand) {
+	g.commands[sub.Name] = sub
+	g.order = append(g.order, sub.Name)
+}
+
+// Usage returns the usage text for this resource group.
+func (g *CommandGroup) Usage() string {
+	var b strings.Builder
+	fmt.Fprintf(&b, "/%s - %s\n", g.Name, g.Description)
+	for _, name := range g.order {
+		sub := g.commands[name]
+		perm := ""
+		if sub.IsWrite {
+			perm = " [owner]"
+		}
+		fmt.Fprintf(&b, "- %s%s\n", sub.Usage, perm)
+	}
+	return b.String()
+}
+
+// Registry holds all registered command groups.
+type Registry struct {
+	groups map[string]*CommandGroup
+	order  []string
+}
+
+func newRegistry() *Registry {
+	return &Registry{
+		groups: make(map[string]*CommandGroup),
+	}
+}
+
+func (r *Registry) RegisterGroup(group *CommandGroup) {
+	r.groups[group.Name] = group
+	r.order = append(r.order, group.Name)
+}
+
+// GlobalHelp returns the top-level help text listing all commands.
+func (r *Registry) GlobalHelp() string {
+	var b strings.Builder
+	b.WriteString("Available commands:\n\n")
+	b.WriteString("/help - Show this help message\n\n")
+	for i, name := range r.order {
+		if i > 0 {
+			b.WriteByte('\n')
+		}
+		group := r.groups[name]
+		b.WriteString(group.Usage())
+	}
+	return strings.TrimRight(b.String(), "\n")
+}