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-remote.test.ts

@@ -10,6 +10,10 @@
 // Cost: ~$0.30-$0.50 per run. Gate-tier (EVALS=1 EVALS_TIER=gate).
 //
 // See setup-gbrain/SKILL.md.tmpl Step 4 (Path 4) for the contract under test.
+// The Step 4 body lives in setup-gbrain/sections/brain-init.md (carved), so
+// the fixture is built via buildSetupGbrainFixture: skeleton + the brain-init
+// and claude-md-persist sections inlined (Step 8 writes the Mode: remote-http
+// block this test asserts on).
 
 import { test, expect } from 'bun:test';
 import { describeE2ETier } from './helpers/e2e-gate';
@@ -18,6 +22,7 @@ import * as os from 'os';
 import * as path from 'path';
 import * as http from 'http';
 import { runAgentSdkTest, passThroughNonAskUserQuestion, resolveClaudeBinary } from './helpers/agent-sdk-runner';
+import { buildSetupGbrainFixture } from './helpers/setup-gbrain-fixture';
 
 // Periodic-tier: the model's interpretation of "follow Path 4 only" is
 // non-deterministic (it sometimes skips Step 8 CLAUDE.md write, sometimes
@@ -144,7 +149,11 @@ describeE2E('/setup-gbrain Path 4 (Remote MCP) — happy path', () => {
     let modelTextOutput = '';
 
     try {
-      const skillPath = path.resolve(import.meta.dir, '..', 'setup-gbrain', 'SKILL.md');
+      // Carve-aware fixture: skeleton + brain-init (Step 4 Path 4 body) +
+      // claude-md-persist (Step 8 block formats), STOP pointers resolved
+      // inline so no Read escapes the sandbox. Non-empty guard inside.
+      const skillPath = path.join(gstackHome, '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' },
         env: childEnv,