fix: echo typed characters in real-time while agent is running

[Desperado]

Summary

• Input was completely invisible while the agent worked — users saw only a status line and had to type blind
• Now each character is echoed live under a › input prompt, with real backspace support (\b \b)
• Enter and Ctrl+C properly advance the cursor off the input line before subsequent output appears
• Tightened first-keystroke trigger from r != 0x1b to r >= 32 — bare Enter/Ctrl+C no longer flash the status line when nothing was typed

Before:

  ⌨ typing — Enter to queue, Ctrl+C to cancel
[invisible typing]
  ✓ queued [1]: run the login tests again

After:

  ⌨ typing — Enter to queue · Ctrl+C to cancel
  › run the login tests again
  ✓ queued [1]: run the login tests again

Test plan

• Start the agent on a long-running task, type a message mid-run — characters should appear after ›
• Backspace should erase characters visually
• Enter should confirm with ✓ queued on the next line
• Ctrl+C should cancel the agent without leaving cursor stranded on the input line
• Pressing Enter or Ctrl+C before typing anything should not flash the status line

🤖 Generated with Claude Code

[qualitymaxapp]

✅ QualityMax Pipeline

Gate	Result
🔍 AI Review	✅ Clean
🧪 Repo Tests	✅ 298/298 passed (go)
🤖 AI Tests	✅ 28/32 passed

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