refactor(annotate): delegate distribution to HF Jobs; drop SLURM/local switch

The executor previously claimed it would "optionally hand off" to
datatrove's LocalPipelineExecutor or SlurmPipelineExecutor — but it
already runs phases inline in every code path, and HF Jobs (see
``examples/annotation/run_hf_job.py``) is the actual distribution
strategy. Stop pretending we have an executor selector.

* `executor.py`: drop `select_executor_class`, the "kind" log line, and
  the references to LocalPipelineExecutor / SlurmPipelineExecutor.
  Module docstring now says distribution is delegated to HF Jobs.
* `config.py`: drop `auto_threshold`, `force_local`, `slurm_partition`,
  `slurm_gpus`, `slurm_time`, `workers`. `ExecutorConfig` keeps only
  `episode_parallelism`. While here, prune the longer "why" docstrings
  on every field down to the load-bearing bits — full story moves to
  `docs/source/annotation_pipeline.mdx`.
* `pyproject.toml`: drop `datatrove>=0.4.0,<2.0.0` from the
  `[annotations]` extra; the dep was only there for the (never used)
  cluster executors. Comment block notes the new HF-Jobs delegation.
* `reader.py`, `lerobot_annotate.py`: drop their own datatrove /
  flavor-namespace mentions.
* `docs/source/annotation_pipeline.mdx`:
  - remove the flavor-namespace / sidecar paragraph (out of scope —
    "multiple revisions = multiple copies" is dataset-level policy);
  - remove the "writer drops the legacy `subtask_index` column" note
    (already covered by PR 1's intentional-break call-out);
  - remove the chat-template + `apply_chat_template(messages, tools=...)`
    line (covered by Tools doc);
  - replace the "executor picks Local vs Slurm" paragraph with
    `--executor.episode_parallelism` and a pointer to HF Jobs;
  - rewrite the style→recipe section to talk about "recipes" generically
    instead of pinning a specific YAML;
  - add a "Running on Hugging Face Jobs" section pointing at
    `examples/annotation/run_hf_job.py`;
  - add a "Running locally" example matching the CLI's docstring
    (`uv run lerobot-annotate --root=... --vlm.model_id=...`);
  - extend the paper-inspirations list with Pi0.7 and Steerable VLA
    Policies (Zhao 2025) for Module 3.

Tests: same 3 pre-existing failures as before this commit (2 module
assertions still in flight; 1 carryover from PR 1). 41/44 pass.
Pre-commit clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lerobot/annotations/steerable_pipeline/reader.py

@@ -26,9 +26,7 @@ episode containing:
 - ``frames_df``: pandas.DataFrame slice for the episode (only loaded on demand)
 
 This shape lets each module operate per-episode without loading all parquet
-rows into memory at once. It deliberately does not depend on datatrove —
-datatrove integration wraps this generator inside a ``PipelineStep`` in
-:mod:`.executor`.
+rows into memory at once.
 """
 
 from __future__ import annotations