feat(mcp): wire brain_context tool

Returns top-N relevant brain entries for a project context. Combines
BM25 hits on project name with 2-hop graph expansion via Track A's
graphstore (when BRAIN_GRAPH_ENABLED). Closes hyperguild#28.

Notes on implementation choices that deviate slightly from the spec:
- Excerpt length: 200 chars per spec (vs the 300 used by search.Result).
  truncateExcerpt clamps the already-stripped BM25 excerpt; graph-only
  neighbours load their excerpt from disk via a private readExcerpt
  helper (search.hydrate is unexported).
- Graph scoring: 0.6 / max(1, distance) per neighbour, so distance-1
  contributes 0.6 and distance-2 contributes 0.3. BM25 hits decay
  linearly from 3.0 (rank-0) to 1.0 (rank-2), giving BM25 hits a
  natural ceiling above pure-graph hits while still letting a doc
  surfaced via both edge types outrank a BM25-only one.
- Test placement: package mcp (internal) rather than mcp_test, because
  graphReader is unexported and WithGraph only accepts *PGStore; an
  internal test can install a dual-interface fake directly on s.graph
  without spinning up postgres.

Bump-Type: minor

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

ingestion/internal/mcp/handlers.go

@@ -122,6 +122,19 @@ func (s *Server) tools() []map[string]any {
 				"depth":     int_("max traversal depth for op=subgraph (default 2, clamped to 6) and op=path (default 4, clamped to 8)"),
 			}),
 		},
+		{
+			"name":        "brain_context",
+			"description": "Return top-N relevant brain entries for a project context. Use at session start or before a complex task to load prior decisions, corrections, and surprises.",
+			"inputSchema": schema([]string{"project_root"}, map[string]any{
+				"project_root": str("absolute path to the project root"),
+				"recent_files": map[string]any{
+					"type":        "array",
+					"items":       map[string]any{"type": "string"},
+					"description": "optional: recent file paths in the project to bias relevance",
+				},
+				"limit": int_("max entries to return, default 10"),
+			}),
+		},
 		{
 			"name":        "session_log",
 			"description": "Append a structured entry to brain/sessions/<session_id>.jsonl.",