Drop versioning vocabulary, polish docstrings, fix world-aware policy resolution

User-facing pass after the project was decided to be a single
submission with no inner iterations.

* Remove every "v1"/"v2"/"versioning" reference from the docs:
  - README mecanum section trims the "v1 predates the rewrite" prose
    in favour of a self-contained retrain recipe.
  - The 3.2 GB `training/runs/v1_clean/` backup directory is deleted.
* Refresh control-layer docstrings:
  - `sheep_tracker.py` header now describes the three actual pipeline
    stages (consensus, prediction, pen latching) instead of layering
    the consensus stage on top of a stale "predictive mode" preamble.
  - `controllers/shepherd_dog/shepherd_dog.py` mode list is
    up-to-date — adds `universal`, removes outdated single-policy
    default paths, mentions `HERDING_USE_GT=1` as the perception
    ablation.
* Refresh training command examples:
  - `training/bc/collect.py` and `training/bc/pretrain.py` usage
    snippets show the world-suffixed paths the Makefile actually
    uses; the `--out` arg is now required so old "demos.npz"
    invocations error loudly instead of silently overwriting.
  - `training/README.md` rewritten — drops the legacy `runs/bc`
    diagram, documents the per-(drive, world) pipeline, and adds
    the mecanum retraining caveat.
* Fix policy-directory resolution end-to-end:
  - `tools/run_webots.sh` now tries
    `training/runs/{bc,rl}_<drive>_<world>` first, then the drive-
    only path, then the bare-mode legacy path — matching the actual
    on-disk layout. Previously it looked for `bc_<drive>` (no
    world) and silently fell back to `bc`, masking the world
    selection.
  - `controllers/shepherd_dog/shepherd_dog.py:_resolve_policy_dir`
    has the same fix plus a latent NameError unmasked: it referenced
    `DRIVE_MODE` before that variable was set at module load. The
    block is restructured so MODE/DRIVE_MODE/WORLD are resolved
    first, then the function uses them as explicit arguments.

126 pytest cases still pass.

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

training/bc/collect.py

@@ -8,8 +8,8 @@ the same partial-obs view the student will have at deployment.
 
 Usage::
 
-    python -m training.bc.collect --teacher strombom \\
-        --out training/bc/demos.npz --frame-stack 4
+    python -m training.bc.collect --teacher universal --drive-mode differential \\
+        --world field --out training/bc/demos_differential_field.npz --frame-stack 4
 """
 
 from __future__ import annotations
@@ -125,7 +125,9 @@ def collect_one(n_sheep: int, seed: int, max_steps: int, subsample: int,
 
 def main():
     parser = argparse.ArgumentParser()
-    parser.add_argument("--out", default="training/bc/demos.npz")
+    parser.add_argument("--out", required=True,
+                        help="Output .npz path (convention: "
+                             "training/bc/demos_<drive>_<world>.npz).")
     parser.add_argument("--n-sheep-list", default="1,2,3,5,8,10")
     parser.add_argument("--seeds-per-n", type=int, default=15)
     parser.add_argument("--max-steps", type=int, default=30000)