feat: migrate eval storage to project-scoped paths

Move eval results and E2E run artifacts from ~/.gstack-dev/evals/ to
~/.gstack/projects/$SLUG/evals/ so each project's eval history lives
alongside its other gstack data. Falls back to legacy path if slug
detection fails.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/eval-compare.ts

@@ -15,10 +15,11 @@ import {
   findPreviousRun,
   compareEvalResults,
   formatComparison,
+  getProjectEvalDir,
 } from '../test/helpers/eval-store';
 import type { EvalResult } from '../test/helpers/eval-store';
 
-const EVAL_DIR = path.join(os.homedir(), '.gstack-dev', 'evals');
+const EVAL_DIR = getProjectEvalDir();
 
 function loadResult(filepath: string): EvalResult {
   // Resolve relative to EVAL_DIR if not absolute