fix(tui): propagate error messages when subagent tasks crash

[echobt]

Summary

When a subagent (Task tool) terminates unexpectedly due to a panic or cancellation, the main agent now receives a proper error message instead of no response at all.

Problem

When a subagent is launched (e.g., 'task research'), it creates a todo list and starts working. However, if the subagent task crashes unexpectedly (panic, timeout, cancellation), the todo list disappears and no message is received by the main agent. This made it impossible to understand what went wrong.

Solution

Added check_crashed_tasks() method that:

• Periodically checks for finished JoinHandles in running_tool_tasks (called on every tick)
• Detects tasks that ended without sending Completed/Failed events
• Extracts panic messages or cancellation status from the JoinHandle
• Sends ToolEvent::Failed with descriptive error message back to main agent

Example Error Messages

• Panic: Subagent panicked: <panic payload>
• Cancelled: Subagent task was cancelled
• Other: Subagent task failed: <error>

Testing

• cargo check -p cortex-tui passes
• cargo fmt -p cortex-tui -- --check passes

[blacksmith-sh]

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