Curation v2 step 4: triage replaces the gate

DeepSeek triage over every eligible article (plan §10) with cached
assessments in article_assessments, the union admission with quotas and
exploration slots (§11), hygiene moved to admit.rs with the churn rule
reading assessments, prefilter.rs reduced to hygiene and text heuristic,
prefilter_keep removed in favour of curation.ranking.deep_keep, the
scores table dropped (migration 0003), and --rescore on generate.

Implemented by Codex (gpt-5.4, high effort) from
docs/plans/curation-v2-briefs/step4.md; reviewed against plan §10–§11.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A1rCLQeKBgnBo3oTgHuTMe

tests/fixtures/deepseek_triage_batch.json

@@ -0,0 +1,16 @@
+{
+  "articles": [
+    {
+      "id": 4821,
+      "interest": 7.5,
+      "kind": "first_hand",
+      "why": "Specific field notes from a difficult migration"
+    },
+    {
+      "id": 4822,
+      "interest": 3.0,
+      "kind": "news",
+      "why": "Competent but familiar daily industry coverage"
+    }
+  ]
+}