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)

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;