refactor(annotate): drop dataset-level `tools` parquet column

PR 2 used to write a top-level ``tools`` column on every parquet shard
holding the JSON schema for the ``say`` tool, broadcast identically
across every row. That extends PR 1's schema for no real information
gain — the schema is a fixed code constant, parquet's RLE/dict encoding
collapses it on disk anyway, and HF/TRL chat-template consumers can
just import the constant directly.

PR 2 should fill in PR 1's existing schema, not add to it. So:

- ``writer.py``: stop emitting the ``tools`` column. Strip any legacy
  ``tools`` column from older shards on rerun so the schema converges to
  v3.1. ``SAY_TOOL_SCHEMA`` stays as a public constant (now joined by
  ``DEFAULT_TOOLS = [SAY_TOOL_SCHEMA]``); chat-template policies and the
  visualizer import them directly.
- ``test_writer.py``: replace the "tools column present" assertion with
  one that explicitly checks the column is absent, plus a new test
  asserting the constant's shape.
- ``test_pipeline_recipe_render.py``: drop the tools-column read; assert
  it's not present in the rewritten parquet.
- ``annotation_pipeline.mdx``: update the writer description to note the
  parquet stays small and the schema lives as a code constant.

If multi-tool-set support ever becomes real (datasets with different
tool inventories), the right home is ``meta/info.json["tools"]`` —
adding it later is non-breaking; ripping out a parquet column already
shipped is not.

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

tests/annotations/test_pipeline_recipe_render.py

@@ -17,7 +17,6 @@
 
 from __future__ import annotations
 
-import json
 from pathlib import Path
 
 import pyarrow.parquet as pq
@@ -130,6 +129,7 @@ def test_pr1_canonical_recipe_renders_nonempty_from_pipeline_output(
     say = speech_rows[0]["tool_calls"][0]
     assert say["function"]["name"] == "say"
     assert isinstance(say["function"]["arguments"]["text"], str)
-    # Tools column carries the say schema
-    tools = json.loads(table.column("tools").to_pylist()[0])
-    assert tools and tools[0]["function"]["name"] == "say"
+    # PR 2 no longer writes a ``tools`` column — the say schema lives as a
+    # constant (``SAY_TOOL_SCHEMA``) so PR 1's row struct is the single
+    # source of truth for the v3.1 schema.
+    assert "tools" not in table.column_names