Add escape key to cancel agent response

[evantahler]

Summary

• Add Escape key binding to cancel streaming agent responses
• Show "[esc] to cancel" hint in the UI while streaming
• Preserve partial responses and display a red cancellation message

Test plan

• Start the agent and press Escape while a response is streaming
• Verify partial response is preserved (if any text was received)
• Verify red "Response cancelled." message appears
• Verify tests pass: bun test
• Verify lint passes: bun lint

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