Add readline-based REPL input with persistent history and arrow-key navigation

[Copilot]

The REPL was using raw stdin line reads, so arrow keys emitted escape sequences and command history was unavailable. This change replaces prompt input handling with rustyline to provide standard line editing, up/down history navigation, and persisted history across sessions.

• REPL input handling

  • Replaced tokio line-based stdin loop in src/main.rs with rustyline::DefaultEditor.
  • Prompt now supports readline-style editing semantics (cursor movement, backspace/delete, etc.).

• History persistence

  • Added default_history_path() in src/consts.rs to centralize history location at ~/.golem/history.txt.
  • REPL now loads history on startup and saves on exit.
  • Uses auto_add_history(true) so non-empty submitted lines are tracked without manual insertion.

• Interrupt / EOF behavior

  • Mapped ReadlineError::Interrupted to prompt interrupt handling (Ctrl+C) without exiting the REPL.
  • Mapped ReadlineError::Eof to clean REPL termination (Ctrl+D), preserving expected terminal behavior.

• Dependencies and docs

  • Added rustyline = "17.0.2" to Cargo.toml.
  • Updated README REPL section to document arrow-key history and persistence path.

let mut repl: DefaultEditor =
    DefaultEditor::with_config(ReadlineConfig::builder().auto_add_history(true).build())?;

let history_path = default_history_path();
let _ = repl.load_history(&history_path);

loop {
    match repl.readline("\ngolem> ") {
        Ok(line) => { /* existing command/task dispatch */ }
        Err(ReadlineError::Interrupted) => continue, // Ctrl+C
        Err(ReadlineError::Eof) => break,            // Ctrl+D
        Err(e) => { eprintln!("input error: {e}"); break; }
    }
}

let _ = repl.save_history(&history_path);

Original prompt

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This section details on the original issue you should resolve

<issue_title>feat: add REPL input history with up/down arrow keys</issue_title>
<issue_description>Currently the REPL reads raw lines from stdin — no history, no arrow key navigation. Pressing up-arrow just prints escape codes.

Add readline-style input history so up/down arrows cycle through previous inputs. This would use a crate like rustyline to replace the raw tokio::io::stdin().lines() reader in main.rs.

Expected behavior:

• Up arrow: previous input
• Down arrow: next input
• History persists across sessions (save to ~/.golem/history.txt or in the SQLite DB)
• Ctrl+C still interrupts, Ctrl+D still exits
• Line editing (left/right, backspace, delete) works naturally</issue_description>

<agent_instructions>implament this. make sure you actually tested it</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes feat: add REPL input history with up/down arrow keys #48

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