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>