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

tests/pi-extension.test.ts

@@ -19,13 +19,23 @@ vi.mock("typebox", () => ({
 vi.mock("@mariozechner/pi-ai", () => ({
 	complete: vi.fn(async (_model: any, _context: any) => ({
 		role: "assistant" as const,
-		content: [{ type: "text", text: "PURPOSE: Test file\nDEPS: none\nCONCEPTS: testing" }],
+		content: [
+			{
+				type: "text",
+				text: "PURPOSE: Test file\nDEPS: none\nCONCEPTS: testing",
+			},
+		],
 	})),
 }));
 
 import extension from "../pi-extension.js";
 
-const CACHE_FILE = join(homedir(), ".cache", "pi-project-map", "llm-cache.json");
+const CACHE_FILE = join(
+	homedir(),
+	".cache",
+	"pi-project-map",
+	"llm-cache.json",
+);
 function clearCache() {
 	if (existsSync(CACHE_FILE)) unlinkSync(CACHE_FILE);
 }
@@ -43,9 +53,16 @@ describe("pi-extension", () => {
 		mockNotify = vi.fn();
 		mockCtx = {
 			cwd: "/home/project",
-			model: { provider: "openai", id: "gpt-4o-mini", api: "openai-completions" },
+			model: {
+				provider: "openai",
+				id: "gpt-4o-mini",
+				api: "openai-completions",
+			},
 			modelRegistry: {
-				getApiKeyAndHeaders: vi.fn(async () => ({ apiKey: "test-key", headers: {} })),
+				getApiKeyAndHeaders: vi.fn(async () => ({
+					apiKey: "test-key",
+					headers: {},
+				})),
 			},
 			ui: { notify: mockNotify },
 		};