feat(mcp): register brain_update + brain_get, extend brain_write handle

Wires the #45 verbs into the MCP surface (all three sites: tools()
descriptors, handleCall dispatch, package doc comment).

- brain_update: supersede-by-slug or full path; rebuilds wing _index and
  re-tunnels cross-wing matches against the new body (idempotent,
  best-effort), re-indexes the graph, returns {id, path, content_hash,
  superseded}.
- brain_get: fetch by id or path (both are the brain-relative handle);
  returns {id, path, content_hash, frontmatter, body}.
- brain_write: return contract extended from {path} to {id, path,
  content_hash} — path kept for backward compat — so the create path
  also yields a stable handle.

id == relPath; content_hash == sha256 of the file bytes. Tests cover the
supersede happy path, missing-target error + no-create, get by id/path,
write handle, and an end-to-end re-embed test that drives the real
vectorstore.Sync re-index after an update.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/internal/mcp/server.go

@@ -1,7 +1,7 @@
 // Package mcp implements an MCP HTTP handler for the ingestion service.
-// Exposed tools: brain_query, brain_write, brain_index, brain_tunnel,
-// brain_ingest, brain_ingest_raw, brain_answer, brain_classify,
-// brain_graph, brain_context, session_log.
+// Exposed tools: brain_query, brain_write, brain_update, brain_get,
+// brain_index, brain_tunnel, brain_ingest, brain_ingest_raw,
+// brain_answer, brain_classify, brain_graph, brain_context, session_log.
 package mcp
 
 import (
@@ -177,6 +177,10 @@ 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_update":
+		return s.brainUpdate(ctx, args)
+	case "brain_get":
+		return s.brainGet(ctx, args)
 	case "brain_index":
 		return s.brainIndex(ctx, args)
 	case "brain_tunnel":