[FEATURE]: Allow sending prompts directly to subagent sessions

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Subagent sessions can currently be opened and inspected, but the web/desktop UI replaces their composer with a disabled message ("Subagent sessions cannot be prompted"). This makes it impossible to continue a conversation with a subagent once it finishes, or to steer it mid-task.

I want to be able to type a follow-up prompt directly into an open subagent session, just like prompting the main session. This mirrors how ChatGPT-style agent loops let you keep talking to a delegated agent. The prompt API already accepts the child session, so the change is UI-only.

• Rendering the normal composer in child sessions
• Keyboard and command-based focus for the child composer
• Automatic follow-up queue behavior stays unchanged