Project-wide cleanup: gitignore, dead code, stale artifacts, README

Repo hygiene pass after a long working session.

Files removed:
* stage1_train.log — runtime training log (~125 KB), shouldn't have
  been tracked.
* training/bc/demos.npz — orphan default-name demos file from before
  the world+drive-suffixed naming convention took over; no script
  references it.
* training/runs/bc_dagger{1,2}_differential_field/policy.zip — failed
  DAgger experiment artifacts. Per `memory/dagger_results.md` the
  whole DAgger experiment hit 0/5 on Webots transfer; these checkpoints
  have no consumers.

Untracked-but-deleted (no git change) — also cleaned from disk:
* Root-level runtime logs (43 *.log files, all unused — gitignored now).
* training/bc/{combined,dagger}*.npz (5 huge demo blobs, 2.6 GB
  reclaimed; not committed).
* training/bc/v1/ (2.6 GB backup of pre-DAgger demos; reclaimed).
* training/runs/at_20260426_*/ (orphan timestamped runs; reclaimed).
* All __pycache__/.

Dead code removed:
* `herding/control/strombom.py::compute_action_debug` — no callers
  anywhere in the repo.
* `herding/control/sequential.py::compute_action_debug` — same.
* `herding/control/universal.py::compute_action_diff` — same.

.gitignore extended to cover:
* All *.log files (training/eval/webots logs are runtime artifacts).
* training/bc/*.npz (re-collectable on demand by `make bc_demos`).
* training/bc/v1/.
* .pytest_cache, *.pyc, .claude/.

README refreshed:
* Mecanum + round-world coverage in the headline.
* Quick-start updated for DRIVE/WORLD-suffixed Makefile targets,
  GT-bypass example, and the mecanum-retrain caveat.
* Layout reflects the actual current tree (config.py, both protos,
  both worlds, all tools).
* Results table replaced with the Webots end-to-end numbers from
  the 2026-05-16 sweep (8/8 diff combos + LiDAR/GT comparison).

Verification: 126 pytest cases still pass (was 126 going in — no
test-coverage regression from the dead-code removal).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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