resolveSubagentNamespace

Lazily resolve a single subagent's execution namespace from checkpoint history. Intended call site: the first scoped useMessages / useToolCalls mount for a subagent whose namespace is still the default tools:<toolCallId>. A fallback for the hydrate-time bulk seed (#seedDiscoveryFromHistory) — most subagents are already promoted by the time a panel opens.

Skips ids already promoted past default-only (SSE replay or a prior resolve). Concurrent calls for the same id share one getHistory walk via #namespaceResolves.