Summary briefing layer: Today pre-summarized, /a is the canonical read

Make summaries the core reading experience (summary-first, source-forward):
- Cycle pre-warms summaries for Today's 7 (idempotent → only new ones hit the LLM).
- /api/brief items carry their cached summary; Today cards (hero + tiles) show it
  inline, so Today reads as a calm briefing.
- Card title/image now open the /a summary page (the canonical artifact), with a
  visible "Full story" link straight to the source on every card (the escape hatch).
- /a gains related-grouping chips + a Copy-link/share control.
- Tighten the summary prompt: original, factual, no quotations / no close paraphrase.
Long tail stays lazy+cached. No article bodies stored. 129 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

goodnews/queries.py

@@ -133,7 +133,8 @@ def brief(conn: sqlite3.Connection, brief_date: str | None = None, limit: int =
 
     rows = conn.execute(
         f"""
-        SELECT bi.rank, bi.selection_reason, {_ARTICLE_COLUMNS}
+        SELECT bi.rank, bi.selection_reason, {_ARTICLE_COLUMNS},
+               (SELECT summary FROM article_summaries WHERE article_id = a.id) AS summary
         FROM daily_briefs b
         JOIN daily_brief_items bi ON bi.brief_id = b.id
         JOIN articles a ON a.id = bi.article_id