feat: GBrain resolver — brain-first lookup and save-to-brain

New scripts/resolvers/gbrain.ts with two resolver functions:
- GBRAIN_CONTEXT_LOAD: search brain for context before skill starts
- GBRAIN_SAVE_RESULTS: save skill output to brain after completion

Placeholders added to 4 thinking skill templates (office-hours,
investigate, plan-ceo-review, retro). Resolves to empty string on
all hosts except gbrain via suppressedResolvers.

GBRAIN suppression added to all 9 non-gbrain host configs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/opencode.ts

@@ -28,6 +28,8 @@ const opencode: HostConfig = {
     { from: '.claude/skills', to: '.opencode/skills' },
   ],
 
+  suppressedResolvers: ['GBRAIN_CONTEXT_LOAD', 'GBRAIN_SAVE_RESULTS'],
+
   runtimeRoot: {
     globalSymlinks: ['bin', 'browse/dist', 'browse/bin', 'gstack-upgrade', 'ETHOS.md'],
     globalFiles: {