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/llm-extract.test.ts

@@ -75,7 +75,7 @@ describe("llm-extract with mock client", () => {
 			},
 		};
 
-		const result = await extractFileLLM(file, mockClient);
+		const result = await extractFileLLM(file, mockClient, tmpdir());
 		expect(result.purpose).toBe("Test file");
 		expect(result.deps).toEqual([]);
 		expect(result.concepts).toContain("testing");