fix(test): close the cross-shard porcelain race that failed Windows CI

Two-part fix for the gen-skill-docs-out-dir isolation-pin failure:

- cookie-import-browser built its scratch cookie DBs inside the TRACKED
  browse/test/fixtures/ dir (created in beforeAll, deleted in afterAll), so
  they flash as untracked files mid-run — a concurrent shard's porcelain
  snapshot caught the window on Windows. The DBs now live in a per-run
  tmpdir; zero source-tree writes.

- gen-skill-docs-out-dir is the free suite's only LIVE porcelain-snapshot
  test, so it joins TREE_MUTATING (the serial quiet window): any concurrent
  transient tree-write can race it, and its own spawned render rewrites
  llms.txt/agents-digest in place (idempotent on a fresh tree).

The race is pre-existing; this branch's +5 test files reshuffled shard
composition and exposed it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/test-free-shards.ts

@@ -407,6 +407,8 @@ export const TREE_MUTATING: Record<string, string> = {
     'golden tests read .agents/.factory artifacts produced by gen-skill-docs.test.ts, and its beforeAll generates them when missing (#2532) — must not race the parallel readers or run before the mutators window',
   'test/catalog-trim.test.ts':
     'imports scripts/gen-skill-docs.ts, whose top-level body regenerates the full claude host at import time (71 files; idempotent on a fresh tree, but a stale tree gets rewritten mid-window) — same hazard class as #2532',
+  'test/gen-skill-docs-out-dir.test.ts':
+    'PORCELAIN READER — its before/after git-status pin needs a quiet tree (a concurrent shard\'s transient fixture write raced it on Windows CI), and its spawned render rewrites llms.txt/agents-digest in place (idempotent on a fresh tree)',
   // Ratchet readers (measure the tree; need it quiet):
   'test/parity-suite.test.ts': 'RATCHET READER — parity caps measure live SKILL.md/section bytes',
   'test/skill-size-budget.test.ts': 'RATCHET READER — per-skill and corpus size budgets measure the live tree',