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.go

@@ -1,5 +1,6 @@
 // Package mcp implements an MCP HTTP handler for the ingestion service.
-// Exposed tools: brain_query, brain_write, brain_ingest, brain_ingest_raw, session_log.
+// Exposed tools: brain_query, brain_write, brain_index, brain_ingest,
+// brain_ingest_raw, brain_answer, brain_classify, session_log.
 package mcp
 
 import (
@@ -136,6 +137,8 @@ func (s *Server) handleCall(ctx context.Context, name string, args json.RawMessa
 		return s.brainQuery(ctx, args)
 	case "brain_write":
 		return s.brainWrite(ctx, args)
+	case "brain_index":
+		return s.brainIndex(ctx, args)
 	case "brain_ingest_raw":
 		return s.brainIngestRaw(ctx, args)
 	case "brain_ingest":