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/server_test.go

@@ -57,7 +57,8 @@ func TestServerToolsList(t *testing.T) {
 	assert.ElementsMatch(t, []string{
 		"brain_query", "brain_write", "brain_index", "brain_tunnel",
 		"brain_ingest_raw", "brain_ingest",
-		"brain_answer", "brain_classify", "brain_graph", "session_log",
+		"brain_answer", "brain_classify", "brain_graph", "brain_context",
+		"session_log",
 	}, names)
 }