feat: Configurable TUI keybindings via config.yaml

[yimeng]

Summary

This PR adds support for customizing keyboard shortcuts in the interactive CLI through the
config.yaml file. This addresses the common issue where Ctrl+D conflicts with macOS terminal
behavior (forward delete character).

Changes

New Configuration Section

Added keybindings section to DEFAULT_CONFIG in hermes_cli/config.py:

keybindings:
  # Exit the application (disabled by default to preserve macOS delete-char behavior)
  exit: null
  # suspend: "ctrl+d"    # Uncomment to enable Ctrl+D exit
  
  # Suspend process to background (Unix only)
  suspend: "ctrl+z"
  
  # Insert newline in multi-line input
  newline: "ctrl+j"
  
  # Alt+Enter also inserts newline
  alt_newline: "escape+enter"

Keybinding Format

• Keys support ctrl+x, alt+x, c-x, a-x formats (case-insensitive)
• Multi-key sequences like escape+enter are supported
• Set any binding to null or empty string to disable it

Modified Bindings

The following bindings are now configurable:

Action	Config Key	Default
Exit Hermes	exit	null (disabled)
Suspend to background	suspend	ctrl+z
Insert newline	newline	ctrl+j
Alt+Enter newline	alt_newline	escape+enter

Implementation Details

• Added helper functions _normalize_key() and _get_keybinding() in cli.py
• Exit keybinding defaults to null to preserve macOS delete-char behavior
• All changes are backward compatible - existing configs work without changes

Testing

1. Test default behavior (exit disabled):

   keybindings:
     exit: null

   Ctrl+D should perform forward delete (macOS default) or EOF (other terminals).

2. Test enabling Ctrl+D exit:

   keybindings:
     exit: "ctrl+d"

   Ctrl+D should exit Hermes.

3. Test custom exit key:

   keybindings:
     exit: "ctrl+\\"  # Ctrl+Backslash

Related Issues

This PR addresses the common user request to disable or rebind Ctrl+D to avoid
conflicts with terminal emulators on macOS.