fix(test): classify catalog-trim.test.ts as tree-mutating

Discovered while landing the duplicate-{{PREAMBLE}} guard: importing
scripts/gen-skill-docs.ts executes its top-level body, which regenerates the
entire claude host (71 GENERATED files) at import time. catalog-trim.test.ts
does that import from a PARALLEL shard — the same read-during-regeneration
hazard class as #2532, invisible only because the regen is byte-identical on
a fresh tree. Move it to the serial tree-mutating window.

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

scripts/test-free-shards.ts

@@ -357,6 +357,8 @@ export const TREE_MUTATING: Record<string, string> = {
     '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',
+  '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',
   // 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',