envs/_template: make template instantiable against real EnvAdapter ABC

The shipped env_template.py and loader_template.py described the same
fictional async execute / evaluate / build_prompt API documented in
docs/reference/api.md. As a result TemplateBenchmarkEnv(cfg) raised
'TypeError: Can't instantiate abstract class' for every copy-and-paste
user who followed the in-tree scaffold.

Rewrite the template so it's a working starting point:

- env_template.py: TemplateBenchmarkEnv(EnvAdapter) now implements all
  five real abstract methods (build_train_env, build_eval_env, rollout,
  reflect, get_task_types) with no-op defaults documented as TODO.
  Instantiable today; pytest 60/60 still passes.
- loader_template.py: TemplateBenchmarkLoader(SplitDataLoader)
  implements load_split_items for .json / .jsonl input and explains the
  optional load_raw_items override for split_mode="ratio".
- README.md: usage steps now point at scripts/train.py's _ENV_REGISTRY
  (the real registry) instead of a non-existent BENCHMARK_REGISTRY in
  skillopt/envs/__init__.py, and link to the rewritten new-benchmark
  guide.
- config_template.yaml: _base_ is a string path (not a list, which the
  loader rejects); skill_init is commented out with a note so the
  template config doesn't reference a file the user hasn't created.

Verified locally: 'from skillopt.envs._template.env_template import
TemplateBenchmarkEnv; TemplateBenchmarkEnv()' succeeds. Refs
microsoft/SkillOpt#30.

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

skillopt/envs/_template/README.md

@@ -4,16 +4,40 @@ This directory provides scaffold files for adding a new benchmark to SkillOpt.
 
 ## Files
 
-- `env_template.py` — Environment adapter template
-- `loader_template.py` — Data loader template
-- `config_template.yaml` — Config file template
+- `env_template.py` — Environment adapter template (subclasses
+  `EnvAdapter`; implements the 5 abstract methods so the file is
+  instantiable out of the box).
+- `loader_template.py` — Data loader template (subclasses
+  `SplitDataLoader`; implements `load_split_items` for `.json`/`.jsonl`).
+- `config_template.yaml` — Config file template.
 
 ## Usage
 
-1. Copy this directory: `cp -r skillopt/envs/_template skillopt/envs/your_benchmark`
-2. Rename files: remove `_template` suffix
-3. Implement the `TODO` sections
-4. Register in `skillopt/envs/__init__.py`
-5. Create config at `configs/your_benchmark/default.yaml`
+1. **Copy the directory:**
+   ```bash
+   cp -r skillopt/envs/_template skillopt/envs/your_benchmark
+   ```
+2. **Rename the files** (drop the `_template` suffix):
+   ```bash
+   cd skillopt/envs/your_benchmark
+   mv env_template.py    adapter.py
+   mv loader_template.py loader.py
+   ```
+   …and inside each file rename the classes
+   (`TemplateBenchmarkEnv → YourBenchmarkAdapter`,
+   `TemplateBenchmarkLoader → YourBenchmarkLoader`)
+   and fix the cross-import in `adapter.py`.
+3. **Implement the TODO blocks** inside `adapter.py:rollout` and the
+   `_normalize_item` helper in `loader.py`. If you want real reflection,
+   uncomment the `run_minibatch_reflect` block in `adapter.py:reflect`.
+4. **Register** the adapter — add a `try / except ImportError` block in
+   `scripts/train.py`'s `_register_builtins()` mapping the registry key
+   to your `YourBenchmarkAdapter` class. There is no
+   `BENCHMARK_REGISTRY` dict in `skillopt/envs/__init__.py`; the live
+   registry is `_ENV_REGISTRY` in `scripts/train.py`.
+5. **Create the config** at `configs/your_benchmark/default.yaml`
+   (start from `config_template.yaml`). `_base_` is a **string path**,
+   not a list.
 
-See the [documentation](../../docs/guide/new-benchmark.md) for the full guide.
+See the [Add a New Benchmark guide](../../../docs/guide/new-benchmark.md)
+for the full step-by-step with a worked `docfaithful` example.