fix: hide cursor during streaming output

[190km]

Summary

• The terminal cursor was always visible during streaming output from AI tools (Codex, Claude, etc.), making the shimmer/wave text look bad
• Implemented should_hide_cursor_for_streaming_output() in pty.rs using the existing last_input_at / last_output_at timestamps to detect active streaming
• The cursor is now hidden only when output arrived within the last 100ms AND the user hasn't typed for 150ms, so it stays visible during normal interactive use
• Added a repaint request in panel.rs so the cursor reappears promptly (within 150ms) once streaming stops

Test plan

• cargo check passes
• All 15 tests pass (cargo test --locked), including the existing streaming_output_hides_cursor test in renderer.rs
• Manual: run an AI tool (e.g. Claude, Codex) that streams output and verify the cursor is hidden during streaming
• Manual: type interactively and verify the cursor remains visible
• Manual: verify the cursor reappears promptly after streaming stops

🤖 Generated with Claude Code