getWorkspace: return a lazy client synchronously #254

getWorkspace(stub) is async, so it can't be assigned to a class field. Think's workspace is a class field, which is why @cloudflare/think 0.21.0 briefly shipped a lazyWorkspace(async () => getWorkspace(stub)) wrapper. We're removing that from Think in a patch; the laziness belongs here.

Today the remote path awaits __getWorkspaceStub(), then useThink, runtime.backends() and hasAssets, before returning a client. Callers often have a promise of the stub too (agents' parentAgent() is async).

Proposal:

• getWorkspace(handle) returns a WorkspaceClient synchronously and accepts handle | Promise<handle>. The remote stub is fetched on first use; RPC promise pipelining means fs calls don't need to wait for it.
• The metadata that's fetched up front becomes lazy. runtime.backends() is the one that matters: createAITools reads it synchronously to build exec, so a lazy client gets no exec today. Making backends() async (and createAITools await it, or build tools from an async factory) would give shared workspaces exec too.
• await getWorkspace(...) keeps working, since awaiting a non-thenable returns it.

Then a Think agent can write:

override workspace = getWorkspace(this.parentAgent(Directory));

Callers today: agents' agent-think and examples/assistant carry a private lazyWorkspace copy until this lands.