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/cli.ts

@@ -128,7 +128,7 @@ async function main() {
 			const entries = discoverProject(targetPath);
 			console.log(`Scanning ${formatCount(entries.length, "directory")}...`);
 			const client = createClientFromArgs(parsed);
-			await initProject(targetPath, { verbose: false, llmClient: client });
+			await initProject(targetPath, { verbose: false, llmClient: client, cacheDir: targetPath });
 			const elapsed = ((Date.now() - start) / 1000).toFixed(1);
 			console.log(
 				`${pc.green("✓")} Generated ${formatCount(entries.length, ".pi-map.md file")} in ${elapsed}s`,
@@ -143,7 +143,7 @@ async function main() {
 				process.exit(1);
 			}
 			const client = createClientFromArgs(parsed);
-			await patchFile(parsed.positional[0], client);
+			await patchFile(parsed.positional[0], client, process.cwd());
 			console.log(`${pc.green("✓")} Patched`);
 			break;
 		}
@@ -179,7 +179,7 @@ async function main() {
 				`Regenerating ${formatCount(entries.length, ".pi-map.md file")}...`,
 			);
 			const client = createClientFromArgs(parsed);
-			await reinitPath(targetPath, { verbose: false, llmClient: client });
+			await reinitPath(targetPath, { verbose: false, llmClient: client, cacheDir: targetPath });
 			const elapsed = ((Date.now() - start) / 1000).toFixed(1);
 			console.log(`${pc.green("✓")} Regenerated in ${elapsed}s`);
 			break;