feat: integrate worktree isolation into E2E test infrastructure

Add createTestWorktree(), harvestAndCleanup(), and describeWithWorktree()
helpers to e2e-helpers.ts. Add harvest field to EvalTestEntry for
eval-store integration. Register lib/worktree.ts as a global touchfile.

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

test/helpers/touchfiles.ts

@@ -200,6 +200,7 @@ export const GLOBAL_TOUCHFILES = [
   'scripts/gen-skill-docs.ts',
   'test/helpers/touchfiles.ts',
   'browse/test/test-server.ts',
+  'lib/worktree.ts',
 ];
 
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