BUG: Subagents don't inherit session-defined external tools

[knifeyspoony]

Parent copilot session can define external tools and use them normally. When spawning agents via task(), these tools are not available.

repro:

1. Start a Copilot session and register one external/session tool, e.g. hello_tool.
2. Verify the parent agent can call hello_tool successfully.
3. Define a custom agent / task child whose tools allowlist includes hello_tool.
4. Ask the parent to use built-in task() to delegate to that child, with instructions: “Call hello_tool.”
5. Observe the child cannot use it: it reports the tool is unavailable, or no external-tool callback fires.

Observed on CLI 1.0.12

[brolnickij]

I can confirm that this issue also occurs in Node.js on version 0.2.1-preview.2