test: full suite runs as N shard processes; scrub spec-sync child env

Two fixes from the wedge-hunt endgame:

1. Full-suite mode switches from one 'bun test --parallel' invocation to
   N concurrent shard PROCESSES, serial within each (the paid runner's
   proven model; N = min(6, cpus-2)). The single-invocation strategy hit
   three distinct Bun 1.3.13 worker pathologies in one day — a segfault
   whose crashed-worker retry wedged the run, a quarantined file's
   still-running file-level hooks stalling a worker, and spawn-heavy
   files hanging under load — and each one stalled the WHOLE invocation.
   Process shards isolate any wedge to its own shard. First full run
   under this model: no wedge, six epilogues, one real failure named.
   WORKER_HOSTILE stays as the paper trail; --parallel remains available
   per-shard for a future Bun.

2. That one real failure: spec-template-sync regenerates SKILL.md via a
   child that inherited the shard process's env — an earlier test's
   GSTACK_*/GBRAIN_* mutations changed generator output (failed in-suite,
   passed solo on an identical tree). The child now gets a scrubbed env:
   generator output must be a function of the templates, not of whichever
   test ran before.

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

test/spec-template-sync.test.ts

@@ -20,6 +20,17 @@ describe('/spec template/generated sync', () => {
       cwd: ROOT,
       encoding: 'utf-8',
       timeout: 120_000,
+      // Scrubbed env: bun test runs a shard's files serially in ONE process,
+      // so an earlier test's env mutations (GSTACK_*/GBRAIN_* detection vars)
+      // leak into inherited process.env and change generator output — this
+      // test failed in-suite while passing solo on an identical tree. The
+      // generator's output must be a function of the templates, not of
+      // whichever test ran before this one.
+      env: {
+        PATH: process.env.PATH ?? '',
+        HOME: process.env.HOME ?? '',
+        TMPDIR: process.env.TMPDIR ?? '',
+      },
     });
     expect(res.status).toBe(0);