Support lazy tool resolution for subagent profiles

[FredKSchott]

Describe the Bug

Subagent tool definitions are resolved when the agent harness is initialized and remain fixed afterward. This becomes difficult when a tool can only be constructed after asynchronous, invocation-time context is available.

The current workaround is to register a placeholder tool up front, then have its execute function load the required context, locate the real tool by name, and delegate to it. This duplicates tool registration concerns, introduces name-based indirection, and can create an advertised-but-unavailable state if the initialized and deferred tool catalogs diverge.

This may be an API limitation rather than a runtime bug, but the required workaround is difficult to discover and reason about.

Expected Behavior

Subagent profiles should support lazy or asynchronous tool resolution, for example:

defineAgentProfile({
  name: "specialist",
  tools: async ({ signal }) => {
    const context = await loadContext(signal);
    return buildTools(context);
  },
});

The resolver would run when the subagent task starts, using invocation-scoped context and cancellation, while static tool arrays remain supported. Tool definitions should then remain stable for that task.

An equivalent operation-scoped API would also work if model-invoked named subagents can receive those tools without placeholder registration.

Steps to Reproduce

1. Define a named subagent whose tool requires asynchronously loaded invocation context.
2. Initialize the parent agent before that context exists.
3. Invoke the named subagent through the built-in task flow.
4. Observe that the subagent can only use tools fixed during harness initialization; later-loaded tools cannot be supplied without registering a placeholder that delegates to the real tool at execution time.