chore: retire the tag-eval harness — it proved the heads system, job done (operator-approved)

The head-vs-centroid eval (#1130) existed to prove the 'frozen embedding +
trained head' spine; the operator accepted the tagging system and dropped the
harness. Removed per rule 22: TagEvalCard + store, /api/tag_eval blueprint,
tag_eval_run ml task, recover-stalled-tag-eval-runs sweep + beat entry,
TagEvalRun model + table (migration 0073), and its tests.

The eval's data loaders + metric helpers were NOT eval-specific — the nightly
heads trainer runs on them — so they moved verbatim to
services/ml/training_data.py (heads.py import updated; behavior unchanged).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

alembic/versions/0073_drop_tag_eval_run.py

@@ -0,0 +1,46 @@
+"""drop tag_eval_run — the head-vs-centroid eval harness is retired
+
+The eval (#1130) existed to prove the heads tagging spine on the operator's own
+data. It did; the operator accepted the system and retired the harness
+(2026-07-02) — card, API, task, model and this table all go. The eval's data
+loaders + metric helpers live on in services/ml/training_data.py, where the
+production heads trainer uses them nightly.
+
+Revision ID: 0073
+Revises: 0072
+Create Date: 2026-07-02
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+revision: str = "0073"
+down_revision: Union[str, None] = "0072"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.drop_index("ix_tag_eval_run_status", table_name="tag_eval_run")
+    op.drop_table("tag_eval_run")
+
+
+def downgrade() -> None:
+    # Recreates the shape from 0056 (data is not restorable).
+    op.create_table(
+        "tag_eval_run",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column("params", postgresql.JSONB(), nullable=False),
+        sa.Column("status", sa.String(length=16), nullable=False,
+                  server_default="running"),
+        sa.Column("started_at", sa.DateTime(timezone=True), nullable=False,
+                  server_default=sa.func.now()),
+        sa.Column("finished_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("report", postgresql.JSONB(), nullable=True),
+        sa.Column("error", sa.Text(), nullable=True),
+        sa.Column("last_progress_at", sa.DateTime(timezone=True),
+                  nullable=True),
+    )
+    op.create_index("ix_tag_eval_run_status", "tag_eval_run", ["status"])