[FEATURE]: Terminal title (OSC 0/2) should distinguish waiting for user input (question/permission) vs active work

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Describe the enhancement you want to request

Context & Gap

Existing proposals (#36294, #43904) discuss exposing working vs. idle status in the terminal window/tab title (OSC 0/2) via a spinner or glyph. Additionally, PR #45347 recently enhanced internal session tabs with status indicators (? for questions, ! for permissions).

However, the external OS terminal window/tab title (OSC 0/2) still has a critical blind spot: it does not distinguish between:

1. The agent actively computing / executing tools in the background.
2. The agent being blocked and waiting for user input (specifically: interactive question tool prompts or permission approval requests).

Proposed Enhancement

When updating the terminal title via OSC 0/2:

• Waiting for user input (pending question tool prompt or permission request): display an attention prefix such as [?] OC | <title> (or [?] / [!]).
• Actively working: display a rotating spinner (e.g. ⠋ OC | <title>) or busy indicator.
• Idle: standard title OC | <title>.

Why this matters

When managing multiple concurrent sessions across terminal tabs or multiplexers (e.g. tmux, Kitty, WezTerm, GNOME Terminal, Windows Terminal):

• Seeing the spinner tells the user: "Agent is still busy, let it run."
• Seeing [?] immediately alerts the user: "Agent is paused waiting for my answer, switch to this tab."

Without this distinction, a user looking at tab titles either has to switch into every tab to check if a question is blocking progress, or mistake a blocked session for an active one and leave it unattended.