Add Ctrl+L screen clear support in REPL

Problem

In a regular terminal, Ctrl+L pushes the current view to the top of the screen (equivalent to running clear). This is a deeply ingrained muscle-memory shortcut for developers. In Claude Code's REPL:

• Ctrl+L is unbound and does nothing
• !clear gets intercepted/suppressed by the REPL input handler
• /clear clears the conversation context, not the terminal screen
• The only workaround is running raw ANSI escape codes, which is not ergonomic

Proposed Solution

1. Add a terminal:clear (or similar) action that sends the standard clear-screen ANSI escape sequence (\033[2J\033[H)
2. Bind Ctrl+L to this action by default, matching standard terminal behavior
3. Alternatively, make it available as a bindable action in ~/.claude/keybindings.json

Context

This is standard behavior in virtually every terminal emulator, shell (bash, zsh, fish), and REPL (Python, Node, etc.). Its absence is a frequent friction point.