fix: isolate each run's outputs in a timestamped subdirectory

[robin-nitschke]

Problem

All runs write into the same out/ directory. A second run piles its
artifacts on top of the previous one — e.g. out/statistics/ and
out/checkpoint/ end up with mixed files from different runs, which makes
per-run analysis unreliable.

Fix

Each run now writes into a timestamped subdirectory, e.g.
out/run_2026-06-22_10-21-05/. Runs are fully isolated and nothing is
overwritten or deleted.

The change is small because the output path is centralized: once
config.out_dir is updated, every writer inherits it (file logger, cost
tracker, statistics tracker, save.pkl, checkpoint/, tree_render/).
TreeSearch reads config.out_dir directly, so the config value is updated
(not just a local variable). Timestamping happens after the
--init / --list-* early returns, so those still use ./out.

Testing

Two consecutive minimal runs each produced their own run_<timestamp>/
folder; the top-level out/ contained no loose artifacts and nothing was
overwritten.