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)

AGENTS.md

@@ -9,6 +9,8 @@ This repo implements the `subagent` pi extension in `index.ts`.
 - Parent-facing tool: `subagent`.
 - Internal child-only tool: `subagent_finalize`.
 - Never register `subagent_finalize` in the parent context; only when `PI_SUBAGENT_CHILD=1`.
+- Subagents are loaded from `~/.pi/subagents/*.md` (user config only).
+- The `subagent` tool is only registered when at least one subagent exists.
 - Subagent sessions are sticky and persisted at:
   `~/.pi/subagent-sessions/<cwd-hash>/<agent>_<sessionId>.jsonl`.
 - Omitting `sessionId` creates a new UUID-backed session.

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) {

src/constants.ts

@@ -1,10 +1,11 @@
 import * as path from "node:path";
 import { fileURLToPath } from "node:url";
+import { homedir } from "node:os";
 
 const SRC_DIR = path.dirname(fileURLToPath(import.meta.url));
 
 export const EXTENSION_DIR = path.dirname(SRC_DIR);
 export const EXTENSION_ENTRY = path.join(EXTENSION_DIR, "index.ts");
-export const PROMPTS_DIR = path.join(EXTENSION_DIR, "prompts");
+export const SUBAGENTS_DIR = path.join(homedir(), ".pi", "subagents");
 export const FINALIZE_TOOL_NAME = "subagent_finalize";
 export const MAX_FINALIZE_RETRIES = 2;

src/prompts.ts

@@ -1,7 +1,7 @@
 import * as fs from "node:fs";
 import * as path from "node:path";
 import { parseFrontmatter } from "@mariozechner/pi-coding-agent";
-import { FINALIZE_TOOL_NAME, PROMPTS_DIR } from "./constants.ts";
+import { FINALIZE_TOOL_NAME, SUBAGENTS_DIR } from "./constants.ts";
 import type { PromptConfig } from "./types.ts";
 import { FinalizeStatus } from "./types.ts";
 
@@ -23,17 +23,17 @@ export function parseToolList(value: unknown): string[] {
   return [];
 }
 
-// Discover Prompts - Load prompt markdown files from this extension's prompts dir.
+// Discover Prompts - Load prompt markdown files from user config dir.
 export function discoverPrompts(): PromptConfig[] {
-  if (!fs.existsSync(PROMPTS_DIR)) return [];
+  if (!fs.existsSync(SUBAGENTS_DIR)) return [];
 
   const prompts: PromptConfig[] = [];
-  const entries = fs.readdirSync(PROMPTS_DIR, { withFileTypes: true });
+  const entries = fs.readdirSync(SUBAGENTS_DIR, { withFileTypes: true });
   for (const entry of entries) {
     if (!entry.name.endsWith(".md")) continue;
     if (!entry.isFile() && !entry.isSymbolicLink()) continue;
 
-    const filePath = path.join(PROMPTS_DIR, entry.name);
+    const filePath = path.join(SUBAGENTS_DIR, entry.name);
     const content = fs.readFileSync(filePath, "utf-8");
     const { frontmatter, body } =
       parseFrontmatter<Record<string, unknown>>(content);