api: questions on long PDFs are answered (retrieval in the digest meta); README and skill

The digest now handles long documents itself (task 02), so the API's refusal
limit is only a safety cap: DefaultPDFMaxQuestionChunks 200 -> 5000 (the 422
message text is unchanged). In v1fetch the digest-meta object gains "retrieval"
and "embedded" next to chunks_used/chunks_total, only when the digest ran
without a fallback. The PDF section explains a long-doc question is answered
first from BM25 while its chunks embed in the background and hybrid on later
questions; skill/SKILL.md gains one sentence. Given tests byte-identical.

Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>

internal/api/api.go

@@ -569,6 +569,8 @@ func (s *Server) v1fetch(w http.ResponseWriter, r *http.Request) {
 					"model":        dr.Model,
 					"chunks_used":  dr.ChunksUsed,
 					"chunks_total": dr.ChunksTotal,
+					"retrieval":    dr.Retrieval,
+					"embedded":     dr.Embedded,
 					"fallback":     false,
 					"cached":       dr.Cached,
 				}