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/server_test.go

@@ -55,7 +55,7 @@ func TestServerToolsList(t *testing.T) {
 		names = append(names, t.(map[string]any)["name"].(string))
 	}
 	assert.ElementsMatch(t, []string{
-		"brain_query", "brain_write", "brain_ingest_raw", "brain_ingest",
+		"brain_query", "brain_write", "brain_index", "brain_ingest_raw", "brain_ingest",
 		"brain_answer", "brain_classify", "session_log",
 	}, names)
 }