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/tools_context_test.go

@@ -0,0 +1,212 @@
+package mcp
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"sort"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/graph"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/graphstore"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// fakeGraph implements graphsync.Store + graphReader so it can be
+// assigned to Server.graph and downcast by brainContext. Only Subgraph
+// is exercised by brain_context today; the rest are no-op satisfiers.
+type fakeGraph struct {
+	subgraph map[string][]graphstore.Neighbor
+}
+
+func (f *fakeGraph) UpsertEntity(_ context.Context, _ graph.Entity) error { return nil }
+func (f *fakeGraph) ReplaceEdgesForDoc(_ context.Context, _ string, _ []graph.Edge) error {
+	return nil
+}
+func (f *fakeGraph) DeleteByDoc(_ context.Context, _ string) error { return nil }
+
+func (f *fakeGraph) Neighbors(_ context.Context, slug, _ string, _ int) ([]graphstore.Neighbor, error) {
+	return f.subgraph[slug], nil
+}
+
+func (f *fakeGraph) Subgraph(_ context.Context, origin string, _ int) ([]graphstore.Neighbor, error) {
+	return f.subgraph[origin], nil
+}
+
+func (f *fakeGraph) Path(_ context.Context, _, _ string, _ int) ([]graphstore.PathStep, error) {
+	return nil, nil
+}
+
+func writeNote(t *testing.T, brainDir, relPath, title, body string) {
+	t.Helper()
+	full := filepath.Join(brainDir, filepath.FromSlash(relPath))
+	require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+	content := "---\ntitle: " + title + "\n---\n\n" + body
+	require.NoError(t, os.WriteFile(full, []byte(content), 0o644))
+}
+
+// callContext runs brainContext directly and decodes the JSON response.
+func callContext(t *testing.T, s *Server, args map[string]any) map[string]any {
+	t.Helper()
+	raw, err := json.Marshal(args)
+	require.NoError(t, err)
+	out, err := s.brainContext(context.Background(), raw)
+	require.NoError(t, err)
+	var resp map[string]any
+	require.NoError(t, json.Unmarshal(out, &resp))
+	return resp
+}
+
+func sortedSlugs(entries []any) []string {
+	slugs := make([]string, 0, len(entries))
+	for _, e := range entries {
+		slugs = append(slugs, e.(map[string]any)["slug"].(string))
+	}
+	sort.Strings(slugs)
+	return slugs
+}
+
+func TestBrainContext_RejectsMissingProjectRoot(t *testing.T) {
+	s := NewServer(t.TempDir(), nil, nil, nil)
+	_, err := s.brainContext(context.Background(), json.RawMessage(`{}`))
+	assert.Error(t, err)
+}
+
+func TestBrainContext_RejectsUnusableBasename(t *testing.T) {
+	s := NewServer(t.TempDir(), nil, nil, nil)
+	_, err := s.brainContext(context.Background(), json.RawMessage(`{"project_root":"/"}`))
+	assert.Error(t, err)
+}
+
+func TestBrainContext_BM25Only_NoGraph(t *testing.T) {
+	brainDir := t.TempDir()
+	// Two notes whose body contains the hyphenated project name. BM25
+	// uses literal substring matching after whitespace tokenisation, so
+	// the bodies must carry "azure-tiger" verbatim, not "Azure tiger".
+	writeNote(t, brainDir, "wiki/finance/decisions/azure-tiger-routing.md",
+		"Azure Tiger Routing", "azure-tiger payment routing decisions.")
+	writeNote(t, brainDir, "wiki/finance/facts/iso20022.md",
+		"Azure Tiger ISO 20022 fields", "azure-tiger maps invoice fields to ISO 20022.")
+
+	s := NewServer(brainDir, nil, nil, nil)
+	// graph is nil — only BM25 hits should appear.
+
+	resp := callContext(t, s, map[string]any{
+		"project_root": "/home/mathias/dev/QKX/azure-tiger",
+	})
+	entries := resp["entries"].([]any)
+	require.NotEmpty(t, entries, "expected at least one BM25 hit on project name")
+
+	for _, e := range entries {
+		entry := e.(map[string]any)
+		assert.Equal(t, "bm25", entry["edge_type"], "no graph configured, every entry must be BM25")
+		assert.NotEmpty(t, entry["slug"])
+		assert.NotEmpty(t, entry["doc_path"])
+	}
+}
+
+func TestBrainContext_BM25PlusGraphExpansion(t *testing.T) {
+	brainDir := t.TempDir()
+	// BM25 seed — body carries the hyphenated project name verbatim.
+	writeNote(t, brainDir, "wiki/finance/decisions/azure-tiger-routing.md",
+		"Azure Tiger Routing", "azure-tiger payment routing decisions.")
+	// Graph neighbour — does NOT match BM25 on "azure-tiger" so it can
+	// only arrive via the graph subgraph traversal.
+	writeNote(t, brainDir, "wiki/finance/facts/sepa-clearing.md",
+		"SEPA Clearing", "SEPA payment clearing rules and timing windows.")
+
+	graphFake := &fakeGraph{
+		subgraph: map[string][]graphstore.Neighbor{
+			"azure-tiger-routing": {
+				{
+					Slug:     "sepa-clearing",
+					Title:    "SEPA Clearing",
+					DocPath:  "wiki/finance/facts/sepa-clearing.md",
+					EdgeType: "wikilink",
+					Distance: 1,
+				},
+			},
+		},
+	}
+	s := NewServer(brainDir, nil, nil, nil)
+	s.graph = graphFake
+
+	resp := callContext(t, s, map[string]any{
+		"project_root": "/home/mathias/dev/QKX/azure-tiger",
+	})
+	entries := resp["entries"].([]any)
+	require.GreaterOrEqual(t, len(entries), 2, "expected BM25 seed plus graph neighbour")
+
+	slugs := sortedSlugs(entries)
+	assert.Contains(t, slugs, "azure-tiger-routing", "BM25 seed must appear")
+	assert.Contains(t, slugs, "sepa-clearing", "graph neighbour must appear")
+
+	// Verify the graph-only entry carries edge_type="graph".
+	var sepaEntry map[string]any
+	for _, e := range entries {
+		m := e.(map[string]any)
+		if m["slug"] == "sepa-clearing" {
+			sepaEntry = m
+			break
+		}
+	}
+	require.NotNil(t, sepaEntry)
+	assert.Equal(t, "graph", sepaEntry["edge_type"])
+	assert.NotEmpty(t, sepaEntry["excerpt"], "excerpt should be loaded from disk for graph neighbours")
+}
+
+func TestBrainContext_LimitClamps(t *testing.T) {
+	brainDir := t.TempDir()
+	// Five notes all matching "azure-tiger".
+	for i, name := range []string{"a", "b", "c", "d", "e"} {
+		writeNote(t, brainDir,
+			"wiki/finance/decisions/azure-tiger-"+name+".md",
+			"Azure Tiger "+name,
+			"azure-tiger note "+name+" with index "+string(rune('0'+i)))
+	}
+	s := NewServer(brainDir, nil, nil, nil)
+	resp := callContext(t, s, map[string]any{
+		"project_root": "/home/mathias/dev/QKX/azure-tiger",
+		"limit":        2,
+	})
+	entries := resp["entries"].([]any)
+	assert.LessOrEqual(t, len(entries), 2)
+}
+
+func TestBrainContext_RecentFilesBoost(t *testing.T) {
+	brainDir := t.TempDir()
+	// Both notes BM25-match the project name, but azure-tiger-z has
+	// twice the term frequency so it naturally ranks above azure-tiger-a.
+	// The recent_files boost on azure-tiger-a should pull it level on
+	// score; the alphabetical slug tiebreaker (a < z) then promotes it
+	// to the top — exercising both the boost and the deterministic
+	// tiebreak.
+	writeNote(t, brainDir, "wiki/finance/decisions/azure-tiger-a.md",
+		"A", "azure-tiger note about a.")
+	writeNote(t, brainDir, "wiki/finance/decisions/azure-tiger-z.md",
+		"Z", "azure-tiger azure-tiger note about z.")
+
+	s := NewServer(brainDir, nil, nil, nil)
+
+	// Baseline ranking: azure-tiger-z must lead (higher term frequency).
+	baseline := callContext(t, s, map[string]any{
+		"project_root": "/home/mathias/dev/QKX/azure-tiger",
+	})
+	baselineEntries := baseline["entries"].([]any)
+	require.GreaterOrEqual(t, len(baselineEntries), 2)
+	baselineTop := baselineEntries[0].(map[string]any)
+	require.Equal(t, "azure-tiger-z", baselineTop["slug"],
+		"sanity: higher tf must rank first without a boost")
+
+	// With boost on azure-tiger-a — boosted entry must now lead.
+	boosted := callContext(t, s, map[string]any{
+		"project_root": "/home/mathias/dev/QKX/azure-tiger",
+		"recent_files": []string{"/some/where/azure-tiger-a.md"},
+	})
+	entries := boosted["entries"].([]any)
+	require.GreaterOrEqual(t, len(entries), 2)
+	top := entries[0].(map[string]any)
+	assert.Equal(t, "azure-tiger-a", top["slug"], "recent_files boost must promote the matching doc")
+}