feat: run the same prompt multiple times into separate numbered output directories

[floo-dck]

Summary

Adds a new --runs CLI flag that executes AutoRecLab multiple times with the
same prompt in a single command. Each run is written to its own numbered
subfolder inside the configured out_dir, so multiple experiment runs no longer
overwrite each other.

Motivation: To analyze the stability/variance of results, we need to run the
exact same prompt several times (e.g. 10x) and compare the outputs.

Changes

• --runs N argument (default 1) in main.py. The prompt is read once and
  reused for every run.
• Numbered output folders: with multiple runs, output goes to
  out/run_01, out/run_02, … The numbering continues from the highest
  existing run_* folder, so previous results are never overwritten. The
  zero-padding adapts to the total run count (e.g. run_01…run_10).
• Refactor: extracted the single-run logic into a run_once() helper and the
  prompt input into get_user_request() for readability.
• Isolated trackers per run: added reset() methods to CostsTracker and
  StatisticsTracker so costs and statistics start clean for every run.
• Per-run logging: attach_file_handler() now removes previously attached
  file handlers, so each run logs to its own debug.log in its own folder.
• README: documented the new --runs option.

Backwards compatibility

With --runs 1 (the default) the behavior is unchanged — output is written
directly to out/ as before.

How to test

uv run main.py --prompt "Build a recommender..." --runs 10