feat: preamble emits gbrain-availability block when capability ok

Extends generate-brain-sync-block.ts to emit Variant A (steady-state, 4
lines) when cwd page_count > 0 or Variant B (empty-corpus emergency, 3
lines) when 0; empty string otherwise. Reads cached page_count from
.gbrain-sync-state.json (handles pretty + compact JSON). Refreshes ship
golden fixtures and bumps the plan-review preamble byte budget to 35K
to absorb the new block.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/gen-skill-docs.test.ts

@@ -316,10 +316,12 @@ describe('gen-skill-docs', () => {
     // (Brain Sync, Context Recovery, Routing Injection are load-bearing
     // functionality, not optional). Budget is set to current size + small
     // headroom; ratchet down if a future slim trims real bytes.
+    // Ratcheted from 33000 → 35000 when the gbrain context-load block was
+    // added to generate-brain-sync-block.ts (per /sync-gbrain plan §4).
     for (const skill of reviewSkills) {
       const content = fs.readFileSync(skill.path, 'utf-8');
       const preamble = extractPreambleBeforeWorkflow(content, skill.markers);
-      expect(Buffer.byteLength(preamble, 'utf-8')).toBeLessThan(33_000);
+      expect(Buffer.byteLength(preamble, 'utf-8')).toBeLessThan(35_000);
     }
   });