refactor: load subagents from user config (~/.pi/subagents/)

- Discover prompts from ~/.pi/subagents/*.md instead of bundled prompts/
- Only register the subagent tool when at least one subagent exists
- Remove directory path from tool description (agents listed dynamically)

index.ts

@@ -38,21 +38,26 @@ export default function (pi: ExtensionAPI) {
     });
   }
 
+  const prompts = discoverPrompts();
+
+  // Skip Registration - Only register the tool if there's at least one subagent.
+  if (prompts.length === 0) return;
+
   pi.registerTool({
     name: "subagent",
     label: "Subagent",
     description:
-      "Delegate a task to a prompt-defined subagent from this extension's prompts/ directory. " +
-      `Available at startup: ${formatPromptList(discoverPrompts())}`,
+      "Delegate a task to a prompt-defined subagent. " +
+      `Available at startup: ${formatPromptList(prompts)}`,
     promptSnippet:
-      "Delegate tasks to subagents by name. Subagent prompts live in prompts/*.md and define approved_tools/denied_tools.",
+      "Delegate tasks to subagents by name. Subagent prompts define approved_tools/denied_tools.",
     parameters: SubagentParams,
 
     async execute(_toolCallId, params, signal, onUpdate, ctx) {
       // Validate Agent
       const validation = validateAgent(
         params.agent,
-        discoverPrompts(),
+        prompts,
         pi.getActiveTools(),
       );
       if (!validation.ok) {