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

frontend/src/components/modal/SuggestionItem.vue

@@ -132,8 +132,8 @@ const hasMenu = computed(() =>
   color: rgb(var(--v-theme-on-surface-variant, var(--v-theme-on-surface)));
   font-family: 'JetBrains Mono', monospace;
 }
-/* Green ✓ / red ✗ verdict pair — same circular language as the eval card
-   (TagEvalCard .fc-act) so accept/reject read identically across surfaces. */
+/* Green ✓ / red ✗ verdict pair — circular buttons so accept/reject read
+   identically across surfaces. */
 .fc-suggestion__acts {
   flex: 0 0 auto; display: flex; gap: 4px;
 }