Curation v2 step 2: Claude Opus 5 editor, the Brief, per-provider budgets

- AnthropicBackend (Messages API, cached system block, output_config.effort,
  server-side fallbacks, refusal surfaced as an error); Llms { bulk, editor }
  with editor_or_bulk(); PriceTable-based UsageMeter per provider.
- [anthropic], [editorial], deepseek.max_concurrent_requests and
  curation.max_article_count config; startup logs resolved providers.
- Budget day is the UTC date of started_at, preloaded from
  runs.provider_costs_json; finish_run writes provider_costs_json and
  config_json. Stage A batches run concurrently with per-batch budget checks.
- Editor prompt with one-line "why" per pick; no minimum lineup size;
  --max-articles is a ceiling; top-up branch deleted; why stored on picks and
  issue_articles.why and rendered in chapters and In this issue.
- Summaries on the editor client (3k-token input, concurrency 4, bulk then
  excerpt fallback); "The Brief" replaces From the Editor; section intros gone.
- Colophon carries per-provider costs and models.

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

tests/fixtures/deepseek_lineup.json

@@ -1,10 +1,10 @@
 {
   "picks": [
-    { "id": 101, "section": "Top Stories", "position": 1, "lead_story": true },
-    { "id": 102, "section": "Tech & Engineering", "position": 1, "lead_story": false },
-    { "id": 103, "section": "Tech & Engineering", "position": 2, "lead_story": false },
-    { "id": 104, "section": "AI & Machine Learning", "position": 1, "lead_story": false },
-    { "id": 105, "section": "Boston & Local", "position": 1, "lead_story": false },
+    { "id": 101, "section": "Top Stories", "position": 1, "lead_story": true, "why": "The migration post-mortem with the invoices still attached" },
+    { "id": 102, "section": "Tech & Engineering", "position": 1, "lead_story": false, "why": "A failover story you'd argue with over coffee" },
+    { "id": 103, "section": "Tech & Engineering", "position": 2, "lead_story": false, "why": "Rare first-hand detail on a tool you use daily" },
+    { "id": 104, "section": "AI & Machine Learning", "position": 1, "lead_story": false, "why": "The one benchmark piece this week that shows its work" },
+    { "id": 105, "section": "Boston & Local", "position": 1, "lead_story": false, "why": "MBTA slow zones charted by a rider, not a press office" },
     { "id": 106, "section": "Culture & Essays", "position": 1 },
     { "section": "Niche Corner", "position": 2, "lead_story": false },
     "the model sometimes trails off like this"