feat(ingestion): add full-text wiki search package

Implements search.Query which walks brainDir/wiki/**/*.md, scores files
by term-frequency across query tokens, and returns results sorted by
score descending. Uses only stdlib — no external search deps.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ingestion/internal/search/search_test.go

@@ -0,0 +1,54 @@
+// ingestion/internal/search/search_test.go
+package search_test
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSearch_ReturnsMatchingPages(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "wiki", "concepts"), 0o755))
+
+	// Write a concept page mentioning "retry"
+	require.NoError(t, os.WriteFile(
+		filepath.Join(dir, "wiki", "concepts", "retry-logic.md"),
+		[]byte("---\ntitle: Retry Logic\ndomain: software\n---\n\nRetry logic handles transient failures by re-attempting operations.\n"),
+		0o644,
+	))
+	// Write an unrelated page
+	require.NoError(t, os.WriteFile(
+		filepath.Join(dir, "wiki", "concepts", "database.md"),
+		[]byte("---\ntitle: Database\ndomain: software\n---\n\nA database stores structured data.\n"),
+		0o644,
+	))
+
+	results, err := search.Query(dir, "retry transient", 5)
+	require.NoError(t, err)
+	require.Len(t, results, 1)
+	assert.Equal(t, "wiki/concepts/retry-logic.md", results[0].Path)
+	assert.Equal(t, "Retry Logic", results[0].Title)
+	assert.Greater(t, results[0].Score, 0)
+	assert.Contains(t, results[0].Excerpt, "Retry")
+}
+
+func TestSearch_RespectsLimit(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "wiki", "concepts"), 0o755))
+	for i := 0; i < 5; i++ {
+		require.NoError(t, os.WriteFile(
+			filepath.Join(dir, "wiki", "concepts", fmt.Sprintf("page-%d.md", i)),
+			[]byte(fmt.Sprintf("---\ntitle: Page %d\n---\n\nThis page mentions retry.\n", i)),
+			0o644,
+		))
+	}
+	results, err := search.Query(dir, "retry", 3)
+	require.NoError(t, err)
+	assert.LessOrEqual(t, len(results), 3)
+}