UX: Multi-line input support in REPL

[evansenter]

Summary

Complex prompts are difficult to enter in the single-line REPL. Support multi-line input for longer prompts.

Proposed Options

Option A: Triple-quote delimiters

> """
Write a function that:
1. Parses JSON
2. Validates the schema
3. Returns typed objects
"""

Option B: Backslash continuation

> Write a function that \
  parses JSON and \
  validates the schema

Option C: Heredoc-style

> <<EOF
Write a function that:
1. Parses JSON
2. Validates the schema  
EOF

Option D: Editor integration

> /edit
# Opens $EDITOR with temp file, submits contents on save

Recommendation

Start with Option A (triple quotes) as it's intuitive and doesn't conflict with shell escapes. Option D is a nice addition for very long prompts.

Implementation Notes

• rustyline supports custom key bindings and multi-line mode
• Could detect opening """ and switch to multi-line mode until closing """

[evansenter]

Implemented partial multiline input support in commit 9358dba:

What's now available:

• Shift+Enter - Insert newline (requires kitty keyboard protocol; works in iTerm2, kitty, WezTerm, alacritty)
• Alt+Enter - Insert newline (fallback for terminals without kitty protocol)
• Bracketed paste - Pasting multiline content no longer auto-submits on each newline

This addresses the core need for entering multiline prompts. The delimiter-based approaches (triple quotes, heredocs) from the original proposal weren't implemented but could be added later if needed.

Leaving this open for potential future enhancements like /edit (open in $EDITOR).