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/mcp/handlers_test.go

@@ -70,6 +70,58 @@ func TestBrainWriteCreatesFile(t *testing.T) {
 	assert.Contains(t, string(got), "# Test")
 }
 
+func TestBrainWriteWingHallRoutesToWiki(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+
+	resp := toolCall(t, srv, "brain_write", map[string]any{
+		"content":  "# Val Vol\n\nbody",
+		"filename": "val-vol-r2",
+		"wing":     "jepa-fx",
+		"hall":     "decisions",
+	})
+	require.Nil(t, resp["error"])
+
+	got, err := os.ReadFile(filepath.Join(brainDir, "wiki", "jepa-fx", "decisions", "val-vol-r2.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "wing: jepa-fx")
+	assert.Contains(t, string(got), "hall: decisions")
+	assert.Contains(t, string(got), "created_at:")
+	assert.Contains(t, string(got), "# Val Vol")
+}
+
+func TestBrainWriteRejectsInvalidHall(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+	resp := toolCall(t, srv, "brain_write", map[string]any{
+		"content": "x",
+		"wing":    "jepa-fx",
+		"hall":    "garbage",
+	})
+	require.NotNil(t, resp["error"])
+}
+
+func TestBrainQueryWingScope(t *testing.T) {
+	brainDir := t.TempDir()
+	for _, p := range []struct{ rel, body string }{
+		{"wiki/jepa-fx/facts/x.md", "---\nwing: jepa-fx\nhall: facts\n---\nfoo keyword.\n"},
+		{"wiki/other/facts/y.md", "---\nwing: other\nhall: facts\n---\nfoo keyword.\n"},
+	} {
+		full := filepath.Join(brainDir, p.rel)
+		require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+		require.NoError(t, os.WriteFile(full, []byte(p.body), 0o644))
+	}
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+	resp := toolCall(t, srv, "brain_query", map[string]any{
+		"query": "foo",
+		"wing":  "jepa-fx",
+	})
+	require.Nil(t, resp["error"])
+	text := resp["result"].(map[string]any)["content"].([]any)[0].(map[string]any)["text"].(string)
+	assert.Contains(t, text, "wiki/jepa-fx/facts/x.md")
+	assert.NotContains(t, text, "wiki/other/facts/y.md")
+}
+
 func TestBrainWriteRejectsTraversal(t *testing.T) {
 	brainDir := t.TempDir()
 	srv := mcp.NewServer(brainDir, nil, nil, nil)