fix(test): host-config goldens self-provision .agents/.factory artifacts

Fixes #2532. The codex/factory golden tests read gitignored artifacts that
only gen-skill-docs.test.ts (serial tree-mutating phase) produces, so the
file failed in isolation and on clean clones (the #2536 "3 failures then 0"
symptom). beforeAll now generates a host's artifacts iff its ship SKILL.md
is missing — never overwriting existing ones, so stale artifacts still fail
the golden. The file is also classified TREE_MUTATING so its provisioning
runs in the serial window, not racing parallel readers.

Verified: full pass with .agents/ and .factory/ deleted (74/74 in isolation).

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

scripts/test-free-shards.ts

@@ -355,6 +355,8 @@ export const TREE_MUTATING: Record<string, string> = {
   'test/skill-validation.test.ts': 'regenerates .agents/ (codex host) artifacts in place (3 sites)',
   'test/gbrain-detection-override.test.ts':
     'regenerates SKILL.md in place with --respect-detection (gbrain variant), then git-restores — readers see inflated skeletons mid-window',
+  'test/host-config.test.ts':
+    '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',
   // 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',