refactor: differentiate reference marker handling between stream and non-stream modes

- Stream: strip both and [reference:N] markers to prevent
  leaking partial link metadata during incremental output
- Non-stream: convert citation/reference markers to Markdown links for
  Claude Messages, Gemini generateContent, and OpenAI Chat/Responses
- Remove StripReferenceMarkers option from call sites; behavior is now
  determined automatically by stream vs non-stream context
- Extend JS runtime stripReferenceMarkersText() to also match [citation:N]
- Add tests for streaming marker stripping and non-stream link conversion

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/textclean/reference_markers.go

@@ -2,19 +2,18 @@ package textclean
 
 import "regexp"
 
-var referenceMarkerPattern = regexp.MustCompile(`(?i)\[reference:\s*\d+\]`)
+var citationReferenceMarkerPattern = regexp.MustCompile(`(?i)\[(citation|reference):\s*\d+\]`)
 
 func StripReferenceMarkers(text string) string {
 	if text == "" {
 		return text
 	}
-	return referenceMarkerPattern.ReplaceAllString(text, "")
+	return citationReferenceMarkerPattern.ReplaceAllString(text, "")
 }
 
-// StripReferenceMarkersEnabled returns true while reference-marker
-// stripping remains the fixed runtime default.  When the behaviour is
-// eventually removed this function can be deleted and callers can drop
-// the conditional.
+// StripReferenceMarkersEnabled returns the default for streaming surfaces,
+// where partial citation/reference markers are hidden before the final
+// link metadata is available.
 func StripReferenceMarkersEnabled() bool {
 	return true
 }