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/pdf_test.go

@@ -396,7 +396,8 @@ func TestPDFQuestionTooLong(t *testing.T) {
 	if msg, _ := out["error"].(string); code != 422 || !strings.Contains(msg, "too long") || !strings.Contains(msg, "grep") {
 		t.Errorf("long question: %d %v; want 422 … too long … grep", code, out)
 	}
-	if DefaultPDFMaxQuestionChunks != 200 {
-		t.Errorf("DefaultPDFMaxQuestionChunks = %d, want 200", DefaultPDFMaxQuestionChunks)
+	// P5b: long documents are answered by retrieval now; the limit is only a safety cap.
+	if DefaultPDFMaxQuestionChunks != 5000 {
+		t.Errorf("DefaultPDFMaxQuestionChunks = %d, want 5000", DefaultPDFMaxQuestionChunks)
 	}
 }