feat: gbrain.context_queries manifests on 6 V1 skills (Lane E partial)

Adds the V1 retrieval contracts. Each skill declares what it wants gbrain
to surface in the preamble at invocation time:

  /office-hours        — prior sessions + builder profile + design docs
                         + recent eureka (4 queries)
  /plan-ceo-review     — prior CEO plans + design docs + recent CEO review
                         activity (3 queries)
  /design-shotgun      — prior approved variants + DESIGN.md + recent
                         design docs (3 queries)
  /design-consultation — existing DESIGN.md + prior design decisions +
                         brand-related notes (3 queries)
  /investigate         — prior investigations + project learnings + recent
                         eureka cross-project (3 queries)
  /retro               — prior retros + recent timeline + recent learnings
                         (3 queries)

Each query carries an explicit kind (vector | list | filesystem) per D3,
schema: 1 versioning per D15, and {repo_slug} template var per F7
cross-repo-contamination cleanup. Mix of vector / list / filesystem
matches what each skill actually needs:

  - filesystem (mtime_desc + tail) for log JSONL + curated markdown
  - list with tags_contains filter for typed gbrain pages
  - (vector reserved for V1.0.1 when gbrain query surface stabilizes)

Smoke test: bun run bin/gstack-brain-context-load.ts --skill-file
office-hours/SKILL.md --repo test-repo --explain returns mode=manifest
queries=4 with the filesystem kinds populating real data from
~/.gstack/builder-profile.jsonl + ~/.gstack/analytics/eureka.jsonl on
this Mac. End-to-end retrieval flow confirmed.

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

retro/SKILL.md

@@ -18,6 +18,25 @@ triggers:
   - weekly retro
   - what did we ship
   - engineering retrospective
+gbrain:
+  schema: 1
+  context_queries:
+    - id: prior-retros
+      kind: filesystem
+      glob: "~/.gstack/projects/{repo_slug}/retros/*.md"
+      sort: mtime_desc
+      limit: 5
+      render_as: "## Prior retros for this project"
+    - id: recent-timeline
+      kind: filesystem
+      glob: "~/.gstack/projects/{repo_slug}/timeline.jsonl"
+      tail: 30
+      render_as: "## Recent timeline events"
+    - id: recent-learnings
+      kind: filesystem
+      glob: "~/.gstack/projects/{repo_slug}/learnings.jsonl"
+      tail: 10
+      render_as: "## Recent learnings"
 ---
 <!-- AUTO-GENERATED from SKILL.md.tmpl — do not edit directly -->
 <!-- Regenerate: bun run gen:skill-docs -->

retro/SKILL.md.tmpl

@@ -18,6 +18,25 @@ triggers:
   - weekly retro
   - what did we ship
   - engineering retrospective
+gbrain:
+  schema: 1
+  context_queries:
+    - id: prior-retros
+      kind: filesystem
+      glob: "~/.gstack/projects/{repo_slug}/retros/*.md"
+      sort: mtime_desc
+      limit: 5
+      render_as: "## Prior retros for this project"
+    - id: recent-timeline
+      kind: filesystem
+      glob: "~/.gstack/projects/{repo_slug}/timeline.jsonl"
+      tail: 30
+      render_as: "## Recent timeline events"
+    - id: recent-learnings
+      kind: filesystem
+      glob: "~/.gstack/projects/{repo_slug}/learnings.jsonl"
+      tail: 10
+      render_as: "## Recent learnings"
 ---
 
 {{PREAMBLE}}