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

backend/app/models/head_training_run.py

@@ -1,7 +1,7 @@
 """HeadTrainingRun — persisted lifecycle of a head-training batch (#114).
 
-Mirrors TagEvalRun so the run SURVIVES navigation and the admin card can show
-live + historical status instead of holding it in transient frontend state.
+A persisted run row (not transient frontend state) so the run SURVIVES
+navigation and the admin card can show live + historical status.
 Training is idempotent (it upserts tag_head rows), so a SIGKILL'd run is harmless
 — a maintenance recovery sweep flips a stalled `running` row to `error`, and the
 next run re-trains. State machine: running → ready / error.
@@ -37,8 +37,8 @@ class HeadTrainingRun(Base):
     n_trained: Mapped[int | None] = mapped_column(Integer, nullable=True)
     n_skipped: Mapped[int | None] = mapped_column(Integer, nullable=True)
     error: Mapped[str | None] = mapped_column(Text, nullable=True)
-    # Last time the task made progress — the recovery sweep tells a live run from
-    # a SIGKILL'd one by this (mirrors TagEvalRun).
+    # Last time the task made progress — the recovery sweep tells a live run
+    # from a SIGKILL'd one by this.
     last_progress_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True
     )