Rewrite CLI REPL with scroll region UI and live steering

[pushkarsingh32]

Summary

• Replace readline-based REPL with custom raw-mode terminal UI using ANSI scroll regions
• Input line is always visible at the bottom while streaming output scrolls above
• Users can type and send steer messages mid-stream without garbling output

Layout

Rows 1..(r-3)  — scroll region (streaming output)
Row  r-2       — queue line (pending steer message in yellow)
Row  r-1       — separator
Row  r         — input line with fake block cursor

Key changes

• Always-on raw mode with manual keystroke handling (no readline)
• Steer support: type during streaming, press Enter to send agent.steer()
• Queue line shows pending steer message, echoes to scroll region when agent picks it up (appears after AI response ends, not inline with streaming text)
• Hidden real cursor with inverse-video fake block cursor on input line
• Proper vertical spacing between user messages and AI responses
• Terminal resize handler redraws all fixed UI elements
• Header line tracking prevents scroll region from overwriting startup info

Test plan

• Build compiles clean
• Manual test: typing during streaming shows on input line without garbling
• Manual test: Enter during streaming sends steer, shown on queue line
• Manual test: steer message appears in scroll region after AI's current response
• Manual test: Ctrl+C aborts during streaming, exits when idle
• Manual test: /quit, /memory, /skills commands work
• Reviewer: test on different terminal sizes and emulators

[vercel]

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[shreyas-lyzr]

This is a large rewrite of the CLI REPL — raw mode, scroll regions, custom input handling. Concerns:

1. Very large diff touching the core REPL loop. This replaces readline entirely with raw terminal management. High risk of regressions on different terminal emulators (iTerm, Windows Terminal, SSH sessions, etc.)
2. No fallback — if raw mode fails (e.g. piped input, CI, non-TTY), the entire CLI breaks. Need a `process.stdin.isTTY` guard with fallback to the current readline approach.
3. The `headerLines` counter is fragile — any log line change breaks the scroll region math.

I'd suggest splitting this: first PR for the steer queue (simple feature), second PR for the scroll region UI (risky, needs testing).