fix(discuss): hard-fail empty articles and skip RAG on seed turn

Discuss flow was hallucinating unrelated content when article
extraction returned empty or RAG pulled in orphan notes that looked
more relevant than the generic seed prompt.

- seed_article_discussion raises EmptyArticleError on empty body;
  briefing and /news routes return 422 instead of staging an empty
  synthetic tool result.
- build_context skips RAG auto-injection when user_message matches
  ARTICLE_DISCUSS_SEED so the article IS the context on turn one;
  follow-up turns keep RAG on.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/fabledassistant/services/article_context.py

@@ -182,6 +182,15 @@ ARTICLE_DISCUSS_SEED = (
 )
 
 
+class EmptyArticleError(Exception):
+    """Raised when an article has no extractable body text.
+
+    Callers (the briefing and /news discuss routes) map this to a 422 so the
+    user sees a clear error instead of a hallucinated summary built from an
+    empty synthetic tool result.
+    """
+
+
 async def seed_article_discussion(
     conv_id: int,
     item: RssItem,
@@ -213,15 +222,30 @@ async def seed_article_discussion(
         article_content = item.context_prepared
     else:
         raw_body = await get_or_fetch_full_article(item) or item.content or ""
+        if not raw_body.strip():
+            # Hard-fail rather than stage an empty synthetic tool result.
+            # An empty `content` field silently tells the model "the article
+            # has nothing in it" and it confabulates from RAG/history. Better
+            # to surface a clean error to the user.
+            logger.warning(
+                "Article discussion aborted: empty body for rss_item %s (%s)",
+                item.id, item.url,
+            )
+            raise EmptyArticleError(
+                "Couldn't extract any readable text from this article."
+            )
         article_content = await prepare_article_context(
             item.title or "", item.url, raw_body, model,
         )
-        if article_content:
-            async with async_session() as session:
-                fresh = await session.get(RssItem, item.id)
-                if fresh is not None:
-                    fresh.context_prepared = article_content
-                    await session.commit()
+        if not article_content.strip():
+            raise EmptyArticleError(
+                "Couldn't extract any readable text from this article."
+            )
+        async with async_session() as session:
+            fresh = await session.get(RssItem, item.id)
+            if fresh is not None:
+                fresh.context_prepared = article_content
+                await session.commit()
 
     synthetic_tool_calls = [{
         "function": "read_article",