Plan mode constraint not propagated to Agent tool subagents

Bug Description

When plan mode is active (via Shift+Tab), using the Agent tool spawns subagents that are not subject to the plan mode constraint. These subagents can freely make file edits and run non-readonly tools, effectively bypassing plan mode entirely.

Steps to Reproduce

1. Activate plan mode (Shift+Tab)
2. Ask Claude to perform a task
3. Claude uses the Agent tool to spawn a subagent
4. The subagent writes files and executes tools despite plan mode being active in the parent session

Expected Behavior

Plan mode constraint should either:

• Propagate to all spawned Agent subagents, or
• Block Agent tool usage entirely while plan mode is active

Actual Behavior

Spawned agents bypass plan mode and make real file edits without user approval.

Impact

Plan mode provides no protection when Claude routes work through the Agent tool, making the feature unreliable as a review gate before implementation.