feat(setup-gbrain): carve the branch-exclusive install paths into sections

Brain-init (Paths 1/2/3/4 bodies), engine remediation, transcript gate, and
CLAUDE.md persist load on demand — at most one install route ever runs.
Skeleton 75.3KB -> 57.0KB; the Step 1 detect and Step 2 path dispatch stay
always-loaded. New buildSetupGbrainFixture helper gives the periodic E2Es
extract-don't-copy fixtures with a non-empty guard; the voyage-code-3 gate
counts scan the tmpl union (the third init site lives in engine-remediation).

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

test/skill-e2e-setup-gbrain-path4-local-pglite.test.ts

@@ -29,6 +29,7 @@ import {
   passThroughNonAskUserQuestion,
   resolveClaudeBinary,
 } from './helpers/agent-sdk-runner';
+import { buildSetupGbrainFixture } from './helpers/setup-gbrain-fixture';
 
 const describeE2E = describeE2ETier('periodic');
 
@@ -166,11 +167,14 @@ describeE2E('/setup-gbrain Path 4 + Step 4.5 Yes → local PGLite for code', ()
     process.env.GBRAIN_MCP_TOKEN = 'gbrain_fake_token_for_test';
 
     try {
-      const skillPath = path.resolve(
-        import.meta.dir,
-        '..',
-        'setup-gbrain',
-        'SKILL.md',
+      // Carve-aware fixture (see test/helpers/setup-gbrain-fixture.ts):
+      // skeleton + brain-init (Step 4 Path 4 body incl. the Step 4d local
+      // PGLite offer this test says Yes to) + claude-md-persist (Step 8 sits
+      // on the walked path to Step 10). Non-empty guard inside the builder.
+      const skillPath = path.join(sandboxHome, 'setup-gbrain-SKILL.md');
+      fs.writeFileSync(
+        skillPath,
+        buildSetupGbrainFixture(['brain-init.md', 'claude-md-persist.md']),
       );
       const result = await runAgentSdkTest({
         systemPrompt: { type: 'preset', preset: 'claude_code' },