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/settings/MaintenancePanel.vue

@@ -28,7 +28,6 @@
         <HeadsCard />
         <GpuAgentCard />
         <AliasTable />
-        <TagEvalCard />
       </div>
     </section>
 
@@ -55,7 +54,6 @@ import MLThresholdSliders from './MLThresholdSliders.vue'
 import HeadsCard from './HeadsCard.vue'
 import GpuAgentCard from './GpuAgentCard.vue'
 import AliasTable from './AliasTable.vue'
-import TagEvalCard from './TagEvalCard.vue'
 import BackupCard from './BackupCard.vue'
 import { useSystemActivityStore } from '../../stores/systemActivity.js'