feat(graph): GraphRAG augment brain_answer with top-hit subgraph

Commit 4 of Track A — the no-shelfware close-out the grill demanded.
brain_answer now folds the 1-hop outgoing neighbourhood of its top
BM25/rerank hit into the LLM's context as a <related> block when
BRAIN_GRAPH_ENABLED is on. With the flag off the prompt is byte-for-
byte identical to the pre-Track-A behaviour, so existing tests still
pass without modification.

The hop list contains slug, edge_type, doc_path — no extra retrieval
pass, no second LLM call, no file reads. The model can ignore the
block when irrelevant; when it adds signal we get GraphRAG for free.

Refs: docs/superpowers/specs/2026-05-homelab-training-graph-next-step.md
in infra repo + grill addendum item "Track A: GraphRAG wiring into
brain_answer is mandatory in same commit chain (no shelfware risk)".

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

ingestion/internal/mcp/tools_answer.go

@@ -96,6 +96,29 @@ func (s *Server) brainAnswer(ctx context.Context, args json.RawMessage) (json.Ra
 		sources = append(sources, r.Path)
 	}
 
+	// GraphRAG augmentation: when the graph is wired, attach the 1-hop
+	// outgoing neighbourhood of the top BM25/rerank hit as an extra
+	// context block. The LLM can ignore it when irrelevant; when the
+	// neighbour adds signal we don't need a second retrieval pass.
+	// Failures are silently skipped — graph is augmentation, not
+	// correctness.
+	if reader, ok := s.graph.(graphReader); ok && len(results) > 0 {
+		topSlug := slugFromPath(results[0].Path)
+		if topSlug != "" {
+			if ns, gerr := reader.Subgraph(ctx, topSlug, 1); gerr == nil && len(ns) > 0 {
+				sb.WriteString("<related>\n")
+				for _, n := range ns {
+					label := n.Title
+					if label == "" {
+						label = n.Slug
+					}
+					fmt.Fprintf(&sb, "- %s (%s) at %s\n", label, n.EdgeType, n.DocPath)
+				}
+				sb.WriteString("</related>\n\n")
+			}
+		}
+	}
+
 	answer, err := s.answerLLM(ctx, answerSystemPrompt, sb.String()+"Question: "+a.Query)
 	if err != nil {
 		return nil, fmt.Errorf("llm: %w", err)
@@ -107,6 +130,25 @@ func (s *Server) brainAnswer(ctx context.Context, args json.RawMessage) (json.Ra
 	})
 }
 
+// slugFromPath converts "wiki/concepts/foo.md" → "foo".
+// Returns "" when path has no .md suffix or empty basename.
+func slugFromPath(path string) string {
+	if path == "" {
+		return ""
+	}
+	// strip directory
+	for i := len(path) - 1; i >= 0; i-- {
+		if path[i] == '/' {
+			path = path[i+1:]
+			break
+		}
+	}
+	if !strings.HasSuffix(path, ".md") {
+		return ""
+	}
+	return strings.TrimSuffix(path, ".md")
+}
+
 type brainClassifyArgs struct {
 	Text string `json:"text"`
 }