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>

retro/SKILL.md.tmpl

@@ -37,6 +37,8 @@ When the user types `/retro`, run this skill.
 - `/retro global` — cross-project retro across all AI coding tools (7d default)
 - `/retro global 14d` — cross-project retro with explicit window
 
+{{GBRAIN_CONTEXT_LOAD}}
+
 ## Instructions
 
 Parse the argument to determine the time window. Default to 7 days if no argument given. All times should be reported in the user's **local timezone** (use the system default — do NOT set `TZ`).
@@ -60,6 +62,16 @@ Usage: /retro [window | compare | global]
 
 {{LEARNINGS_SEARCH}}
 
+### Non-git context (optional)
+
+Check for non-git context that should be included in the retro:
+
+```bash
+[ -f ~/.gstack/retro-context.md ] && echo "RETRO_CONTEXT_FOUND" || echo "NO_RETRO_CONTEXT"
+```
+
+If `RETRO_CONTEXT_FOUND`: read `~/.gstack/retro-context.md`. This file is user-authored and may contain meeting notes, calendar events, decisions, and other context that doesn't appear in git history. Incorporate this context into the retro narrative where relevant.
+
 ### Step 1: Gather Raw Data
 
 First, fetch origin and identify the current user:
@@ -281,6 +293,8 @@ For each contributor (including the current user), compute:
 
 {{LEARNINGS_LOG}}
 
+{{GBRAIN_SAVE_RESULTS}}
+
 ### Step 10: Week-over-Week Trends (if window >= 14d)
 
 If the time window is 14 days or more, split into weekly buckets and show trends: