feat: move LLM cache to project-local directory

- Cache now lives in <project>/.pi-project-map/cache/llm-cache.json
- Removed global ~/.cache/pi-project-map/ usage
- Cache travels with the project, no cross-project collisions
- Easy to invalidate: rm -rf .pi-project-map/cache/

Files changed:
- llm-cache.ts: accept cacheDir parameter, default to project-local
- llm-extract.ts: pass cacheDir through to cache functions
- init.ts: pass rootPath as cacheDir
- patch.ts: accept and pass cacheDir
- cli.ts: pass targetPath as cacheDir
- pi-extension.ts: pass ctx.cwd as cacheDir
- Tests updated to use temp dirs for cache isolation

src/patch.ts

@@ -13,6 +13,7 @@ const SMALL_PACKAGE_THRESHOLD = 10;
 export async function patchFile(
 	filePath: string,
 	llmClient?: LLMClient,
+	cacheDir?: string,
 ): Promise<void> {
 	const dirPath = dirname(filePath);
 	const mapPath = join(dirPath, ".pi-map.md");
@@ -42,6 +43,7 @@ export async function patchFile(
 				files,
 			},
 			llmClient,
+			cacheDir,
 		);
 		console.log(
 			`Full rewrite of ${mapPath} (small package: ${allFiles.length} files)`,
@@ -49,7 +51,7 @@ export async function patchFile(
 	} else {
 		// Section-level patch
 		const existing = parsePackageMap(readFileSync(mapPath, "utf8"));
-		const llmData = await extractFileLLM(filePath, llmClient);
+		const llmData = await extractFileLLM(filePath, llmClient, cacheDir);
 		const astData = await extractFileAST(filePath);
 		const fileName = basename(filePath);
 		const updatedFile = mergeFileData(fileName, llmData, astData);