feat(brain): structured wing/hall taxonomy + obsidian-compatible layout

Adds a two-dimensional address (wing, hall) to brain notes. A wing is a
topic domain (e.g. jepa-fx, hyperguild); a hall is one of a closed
vocabulary of memory types (facts, decisions, failures, hypotheses,
sources). Notes route to brain/wiki/<wing>/<hall>/<slug>.md with
wing/hall/created_at YAML frontmatter, making the directory a valid
Obsidian vault.

Changes:
- new package ingestion/internal/brain (NotePath, ValidHalls, Sanitise,
  BuildWingIndex, BuildAllWingIndexes)
- api.WriteNote refactored to WriteNoteOptions; wing+hall routes to
  brain/wiki/, otherwise falls back to brain/knowledge/ (legacy)
- search.Query → QueryOptions with optional Wing/Hall filtering; Result
  carries wing/hall extracted from frontmatter or path segments
- MCP tools brain_write and brain_query gain optional wing/hall params
  (hall enum-validated); new brain_index tool regenerates _index.md MOC
- POST /index REST endpoint mirrors brain_index
- brain_write auto-rebuilds the wing's _index.md after a wing+hall write
- scripts/migrate-brain-halls.sh migrates flat brain/wiki/{concepts,entities}/
  into the new layout (dry-run by default, --commit applies)

All existing tests pass; new tests cover wing/hall write routing, scope
filtering, invalid hall rejection, _index.md generation, and migration
script paths.

Closes hyperguild#1.

ingestion/internal/brain/index_test.go

@@ -0,0 +1,85 @@
+package brain_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBuildWingIndex(t *testing.T) {
+	dir := t.TempDir()
+	for _, p := range []struct{ rel, body string }{
+		{"wiki/jepa-fx/decisions/val-vol.md", "---\ntitle: Val Vol R2\ncreated_at: 2026-05-06T10:00:00Z\n---\nbody\n"},
+		{"wiki/jepa-fx/facts/architecture.md", "---\ntitle: Architecture\ncreated_at: 2026-05-04T10:00:00Z\n---\nbody\n"},
+		{"wiki/jepa-fx/sources/paper.md", "---\n---\nbody\n"},
+	} {
+		full := filepath.Join(dir, p.rel)
+		require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+		require.NoError(t, os.WriteFile(full, []byte(p.body), 0o644))
+	}
+
+	require.NoError(t, brain.BuildWingIndex(dir, "jepa-fx"))
+
+	got, err := os.ReadFile(filepath.Join(dir, "wiki", "jepa-fx", "_index.md"))
+	require.NoError(t, err)
+	s := string(got)
+	assert.Contains(t, s, "# jepa-fx")
+	assert.Contains(t, s, "| Hall | Note | Created |")
+	assert.Contains(t, s, "| decisions | [Val Vol R2](decisions/val-vol.md) | 2026-05-06 |")
+	assert.Contains(t, s, "| facts | [Architecture](facts/architecture.md) | 2026-05-04 |")
+	assert.Contains(t, s, "| sources | [paper](sources/paper.md) |")
+	// Halls sorted alphabetically.
+	assert.Less(t, indexOf(s, "decisions"), indexOf(s, "facts"))
+	assert.Less(t, indexOf(s, "facts"), indexOf(s, "sources"))
+}
+
+func TestBuildWingIndex_SkipsInvalidHalls(t *testing.T) {
+	dir := t.TempDir()
+	wingDir := filepath.Join(dir, "wiki", "jepa-fx")
+	require.NoError(t, os.MkdirAll(filepath.Join(wingDir, "garbage"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(wingDir, "garbage", "x.md"), []byte("x"), 0o644))
+	require.NoError(t, os.MkdirAll(filepath.Join(wingDir, "facts"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(wingDir, "facts", "y.md"), []byte("y"), 0o644))
+
+	require.NoError(t, brain.BuildWingIndex(dir, "jepa-fx"))
+	got, err := os.ReadFile(filepath.Join(wingDir, "_index.md"))
+	require.NoError(t, err)
+	s := string(got)
+	assert.Contains(t, s, "facts")
+	assert.NotContains(t, s, "garbage")
+}
+
+func TestBuildAllWingIndexes(t *testing.T) {
+	dir := t.TempDir()
+	for _, p := range []struct{ rel, body string }{
+		{"wiki/a/facts/x.md", "x"},
+		{"wiki/b/facts/y.md", "y"},
+	} {
+		full := filepath.Join(dir, p.rel)
+		require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+		require.NoError(t, os.WriteFile(full, []byte(p.body), 0o644))
+	}
+	require.NoError(t, brain.BuildAllWingIndexes(dir))
+	_, err := os.Stat(filepath.Join(dir, "wiki", "a", "_index.md"))
+	require.NoError(t, err)
+	_, err = os.Stat(filepath.Join(dir, "wiki", "b", "_index.md"))
+	require.NoError(t, err)
+}
+
+func TestBuildWingIndex_NoWingDir(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, brain.BuildWingIndex(dir, "ghost"))
+}
+
+func indexOf(s, sub string) int {
+	for i := 0; i+len(sub) <= len(s); i++ {
+		if s[i:i+len(sub)] == sub {
+			return i
+		}
+	}
+	return -1
+}