Add dynamic prompt text support to REPL

Implements customizable prompt text that can be static or dynamically generated,
appearing before the prompt symbol in the REPL interface.

Changes:
- Add History.get_all/0 to retrieve command history as string list
- Add Arca.Cli.prompt_text/1 to handle prompt generation with three modes:
  - nil config: default prompt (backward compatible)
  - static string: prepends text to default prompt format
  - function/1: delegates full prompt generation to custom function
- Update REPL.repl_prompt/0 to build context and use prompt_text/1

The prompt_text function receives context with config_domain, history_count,
history_cmds, and prompt_symbol. When a function is configured, it has full
control over the entire prompt string generation.

This allows dependent projects to display contextual information in their
prompts while maintaining backward compatibility for existing projects.

Also removes .backup directory from git tracking and adds to .gitignore.

Author: matthewsinclair