refactor: simplify cache location to .cache/llm-cache.json

- Cache now lives at <project>/.cache/llm-cache.json (single hidden dir)
- Remove extra pi-project-map subdirectory level
- Use dirname() instead of string slicing for robustness

src/llm/llm-cache.ts

@@ -5,10 +5,9 @@ import {
 	mkdirSync,
 	renameSync,
 } from "fs";
-import { join } from "path";
+import { dirname, join } from "path";
 
-const DEFAULT_CACHE_SUBDIR = ".cache/pi-project-map";
-const DEFAULT_CACHE_FILE = "llm-cache.json";
+const CACHE_FILE = "llm-cache.json";
 
 interface CacheEntry {
 	result: string;
@@ -16,15 +15,12 @@ interface CacheEntry {
 }
 
 function getCachePath(cacheDir?: string): string {
-	if (cacheDir) {
-		return join(cacheDir, DEFAULT_CACHE_FILE);
-	}
-	// Fallback to cwd when no project root provided
-	return join(process.cwd(), DEFAULT_CACHE_SUBDIR, DEFAULT_CACHE_FILE);
+	const base = cacheDir || process.cwd();
+	return join(base, ".cache", CACHE_FILE);
 }
 
 function ensureCacheDir(cachePath: string): void {
-	const dir = cachePath.substring(0, cachePath.lastIndexOf("/"));
+	const dir = dirname(cachePath);
 	if (!existsSync(dir)) {
 		mkdirSync(dir, { recursive: true });
 	}