[P2] Early input capture during REPL init / 启动期间用户输入捕获

[doudouOUC]

Priority: P2
Category: performance, ui
Parent: #3011

Problem / 问题

User keystrokes typed during REPL initialization (200-500ms) are lost. Users must wait for the prompt to appear before typing.

REPL 初始化期间（200-500ms）用户输入丢失，必须等待提示符出现后才能输入。

Proposed Solution / 方案

1. Add packages/cli/src/utils/earlyInput.ts:
   • startCapturingEarlyInput(): enables raw stdin buffering
   • drainEarlyInput(): returns buffered input and stops capturing
2. Call startCapturingEarlyInput() at the very beginning of main() (before any async work)
3. Pass buffered input to REPL on render

Reference: Claude Code src/utils/earlyInput.ts — same pattern.

Acceptance Criteria / 验收标准

• Keystrokes typed during startup appear in REPL input after render
• No input loss for fast typists
• No impact on non-interactive mode (stdin pipe)

[xxih]

I'd like to work on this. Plan: add an early stdin buffer utility, start capture before async startup work in gemini.tsx, drain buffered input into the interactive REPL only, and add unit tests for TTY/non-TTY behavior.