Allow extended context ([1m]) in Agent tool model parameter

When running a session with claude --model opus[1m], subagents spawned via the Agent tool still get the default 200K context.

The model parameter on Agent only accepts "sonnet", "opus", "haiku" — there's no way to specify opus[1m] or sonnet[1m].

Use case: Academic projects with large CLAUDE.md files, extensive rules, and long system prompts that exceed 200K when combined with agent instructions. Subagents need the extended context to function correctly in these environments.

Proposed fix: Either:

1. Allow model: "opus[1m]" in the Agent tool's model parameter, or
2. Add a global setting to propagate the parent session's context window size to subagents