feat(rag): show cited sources and exit cleanly on interrupt/errors

- After a synthesized answer, print a legend mapping the [n] tags the
  model actually cited to their source files (on stderr, so piping the
  answer stays clean). Only cited tags are listed, not every retrieved
  chunk.
- Catch KeyboardInterrupt/BrokenPipeError for a quiet abort and collapse
  other exceptions to a one-line message; set RAG_DEBUG=1 for the full
  traceback.

packages/rag/default.nix

@@ -16,6 +16,7 @@ writers.writePython3Bin "rag"
     import hashlib
     import os
     import pathlib
+    import re
     import sys
 
     from openai import OpenAI
@@ -128,12 +129,24 @@ writers.writePython3Bin "rag"
             {"role": "user", "content": f"Context:\n{context}\n\nQuestion: {args.text}"},
         ]
         stream = client.chat.completions.create(model=CHAT_MODEL, messages=messages, stream=True)
+        answer = []
         for ev in stream:
             delta = ev.choices[0].delta.content
             if delta:
+                answer.append(delta)
                 sys.stdout.write(delta)
                 sys.stdout.flush()
         sys.stdout.write("\n")
+        # Legend for only the [n] tags actually cited (stderr keeps stdout pipe-clean).
+        cited = sorted({int(n) for n in re.findall(r"\[(\d+)\]", "".join(answer))})
+        cited = [i for i in cited if 1 <= i <= len(hits)]
+        if cited:
+            print("\nQuellen:", file=sys.stderr)
+            for i in cited:
+                h = hits[i - 1]
+                src = h.payload.get("source", "?")
+                chunk = h.payload.get("chunk", "?")
+                print(f"  [{i}] {src}#{chunk}  ({h.score:.3f})", file=sys.stderr)
 
 
     def main():
@@ -157,5 +170,15 @@ writers.writePython3Bin "rag"
 
 
     if __name__ == "__main__":
-        main()
+        try:
+            main()
+        except KeyboardInterrupt:
+            sys.exit(130)
+        except BrokenPipeError:
+            sys.exit(0)
+        except Exception as e:
+            if os.environ.get("RAG_DEBUG"):
+                raise
+            print(f"rag: {type(e).__name__}: {e}", file=sys.stderr)
+            sys.exit(1)
   ''