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>

investigate/SKILL.md.tmpl

@@ -45,6 +45,8 @@ Fixing symptoms creates whack-a-mole debugging. Every fix that doesn't address r
 
 ---
 
+{{GBRAIN_CONTEXT_LOAD}}
+
 ## Phase 1: Root Cause Investigation
 
 Gather context before forming any hypothesis.
@@ -61,6 +63,8 @@ Gather context before forming any hypothesis.
 
 4. **Reproduce:** Can you trigger the bug deterministically? If not, gather more evidence before proceeding.
 
+5. **Check investigation history:** Search prior learnings for investigations on the same files. Recurring bugs in the same area are an architectural smell. If prior investigations exist, note patterns and check if the root cause was structural.
+
 {{LEARNINGS_SEARCH}}
 
 Output: **"Root cause hypothesis: ..."** — a specific, testable claim about what is wrong and why.
@@ -186,8 +190,16 @@ Status:          DONE | DONE_WITH_CONCERNS | BLOCKED
 ════════════════════════════════════════
 ```
 
+Log the investigation as a learning for future sessions. Use `type: "investigation"` and include the affected files so future investigations on the same area can find this:
+
+```bash
+~/.claude/skills/gstack/bin/gstack-learnings-log '{"skill":"investigate","type":"investigation","key":"ROOT_CAUSE_KEY","insight":"ROOT_CAUSE_SUMMARY","confidence":9,"source":"observed","files":["affected/file1.ts","affected/file2.ts"]}'
+```
+
 {{LEARNINGS_LOG}}
 
+{{GBRAIN_SAVE_RESULTS}}
+
 ---
 
 ## Important Rules