feat: implement custom slash commands system

Adds a comprehensive slash command system that allows users to create and manage custom commands:

- Backend implementation in Rust for discovering, loading, and managing slash commands
- Support for both user-level (~/.claude/commands/) and project-level (.claude/commands/) commands
- YAML frontmatter support for command metadata (description, allowed-tools)
- Command namespacing with directory structure (e.g., /namespace:command)
- Detection of special features: bash commands (\!), file references (@), and arguments ($ARGUMENTS)

Frontend enhancements:
- SlashCommandPicker component with autocomplete UI and keyboard navigation
- SlashCommandsManager component for CRUD operations on commands
- Integration with FloatingPromptInput to trigger picker on "/" input
- Visual indicators for command features (bash, files, arguments)
- Grouped display by namespace with search functionality

API additions:
- slash_commands_list: Discover all available commands
- slash_command_get: Retrieve specific command by ID
- slash_command_save: Create or update commands
- slash_command_delete: Remove commands

This implementation provides a foundation for users to create reusable command templates and workflows. Commands are stored as markdown files with optional YAML frontmatter for metadata.

Addresses #127 and #134

src/components/Settings.tsx

@@ -24,6 +24,7 @@ import { Toast, ToastContainer } from "@/components/ui/toast";
 import { ClaudeVersionSelector } from "./ClaudeVersionSelector";
 import { StorageTab } from "./StorageTab";
 import { HooksEditor } from "./HooksEditor";
+import { SlashCommandsManager } from "./SlashCommandsManager";
 
 interface SettingsProps {
   /**
@@ -357,12 +358,13 @@ export const Settings: React.FC<SettingsProps> = ({
       ) : (
         <div className="flex-1 overflow-y-auto p-4">
           <Tabs value={activeTab} onValueChange={setActiveTab} className="w-full">
-            <TabsList className="grid grid-cols-6">
+            <TabsList className="grid grid-cols-7 w-full">
               <TabsTrigger value="general">General</TabsTrigger>
               <TabsTrigger value="permissions">Permissions</TabsTrigger>
               <TabsTrigger value="environment">Environment</TabsTrigger>
               <TabsTrigger value="advanced">Advanced</TabsTrigger>
               <TabsTrigger value="hooks">Hooks</TabsTrigger>
+              <TabsTrigger value="commands">Commands</TabsTrigger>
               <TabsTrigger value="storage">Storage</TabsTrigger>
             </TabsList>
             
@@ -705,6 +707,13 @@ export const Settings: React.FC<SettingsProps> = ({
               </Card>
             </TabsContent>
             
+            {/* Commands Tab */}
+            <TabsContent value="commands">
+              <Card className="p-6">
+                <SlashCommandsManager className="p-0" />
+              </Card>
+            </TabsContent>
+            
             {/* Storage Tab */}
             <TabsContent value="storage">
               <StorageTab />