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  superseded}.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_test.go

@@ -55,7 +55,8 @@ func TestServerToolsList(t *testing.T) {
 		names = append(names, t.(map[string]any)["name"].(string))
 	}
 	assert.ElementsMatch(t, []string{
-		"brain_query", "brain_write", "brain_index", "brain_tunnel",
+		"brain_query", "brain_write", "brain_update", "brain_get",
+		"brain_index", "brain_tunnel",
 		"brain_ingest_raw", "brain_ingest",
 		"brain_answer", "brain_classify", "brain_graph", "brain_context",
 		"session_log",