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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