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/path_test.go

@@ -0,0 +1,73 @@
+package brain_test
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNotePath_Valid(t *testing.T) {
+	got, err := brain.NotePath("/b", "jepa-fx", "decisions", "val-vol-r2")
+	require.NoError(t, err)
+	assert.Equal(t, filepath.Join("/b", "wiki", "jepa-fx", "decisions", "val-vol-r2.md"), got)
+}
+
+func TestNotePath_StripsMdSuffix(t *testing.T) {
+	got, err := brain.NotePath("/b", "x", "facts", "note.md")
+	require.NoError(t, err)
+	assert.Equal(t, filepath.Join("/b", "wiki", "x", "facts", "note.md"), got)
+}
+
+func TestNotePath_SanitisesWingAndSlug(t *testing.T) {
+	got, err := brain.NotePath("/b", "Jepa FX!", "facts", "Val Vol R2")
+	require.NoError(t, err)
+	assert.Equal(t, filepath.Join("/b", "wiki", "jepa-fx", "facts", "val-vol-r2.md"), got)
+}
+
+func TestNotePath_RejectsInvalidHall(t *testing.T) {
+	_, err := brain.NotePath("/b", "x", "garbage", "y")
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "invalid hall")
+}
+
+func TestNotePath_RejectsEmptyWing(t *testing.T) {
+	_, err := brain.NotePath("/b", "!!!", "facts", "y")
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "invalid wing")
+}
+
+func TestNotePath_RejectsEmptySlug(t *testing.T) {
+	_, err := brain.NotePath("/b", "x", "facts", "!!!")
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "invalid slug")
+}
+
+func TestSanitise(t *testing.T) {
+	cases := map[string]string{
+		"Jepa-FX":         "jepa-fx",
+		"  foo  bar  ":    "foo-bar",
+		"Val/Vol\\R2.md":  "val-vol-r2-md",
+		"!!!":             "",
+		"___leading":      "leading",
+		"trailing___":     "trailing",
+		"multi---hyphen":  "multi-hyphen",
+		"UPPER 123 mixed": "upper-123-mixed",
+	}
+	for in, want := range cases {
+		t.Run(in, func(t *testing.T) {
+			assert.Equal(t, want, brain.Sanitise(in))
+		})
+	}
+}
+
+func TestIsValidHall(t *testing.T) {
+	for _, h := range []string{"facts", "decisions", "failures", "hypotheses", "sources"} {
+		assert.True(t, brain.IsValidHall(h), h)
+	}
+	for _, h := range []string{"", "Facts", "facts ", "rooms", "concepts", "entities"} {
+		assert.False(t, brain.IsValidHall(h), h)
+	}
+}