fix: stripMarkers was suppressing all content inside fences

The inFence state machine was dropping all lines between fence
delimiters, not just the delimiters themselves. Tool output, assistant
code blocks — everything inside fences was being eaten.

Fix: remove only [[[...]]] marker lines and bare fence delimiter lines
(``` / ```lang). Preserve all content between them.

fs/handlers_agent.go

@@ -139,12 +139,12 @@ func streamAgentChatText(ctx HandlerCtx, cctx context.Context, base string) ([]b
 	}
 }
 
-// stripMarkers removes [[[...]]] marker lines and source fence lines from raw
-// chat data. Partial lines (no trailing \n) are always passed through — they
-// are streaming content, never markers.
+// stripMarkers removes [[[...]]] marker lines and source fence delimiter lines
+// from raw chat data. Content inside fences is preserved — only the ``` lines
+// themselves are stripped. Partial lines (no trailing \n) are always passed
+// through — they are streaming content, never markers.
 func stripMarkers(data []byte) []byte {
 	var out []byte
-	inFence := false
 	for len(data) > 0 {
 		// Find next newline
 		nl := bytes.IndexByte(data, '\n')
@@ -162,28 +162,15 @@ func stripMarkers(data []byte) []byte {
 			continue
 		}
 
-		// Drop fence open/close lines (``` with optional language suffix)
+		// Drop bare fence delimiter lines (``` or ```lang).
+		// These are the source fences wrapping tool output.
+		// Keep fences with 4+ backticks (````), those are user/assistant markdown.
 		stripped := bytes.TrimSpace(trimmed)
-		if bytes.HasPrefix(stripped, []byte("```")) {
-			if inFence {
-				inFence = false
-				continue
-			}
-			// Opening fence — only drop if it's a source fence (tool output),
-			// not markdown fences in assistant prose. Tool fences follow
-			// immediately after a marker line, so if we just dropped a marker
-			// and this is the next line, it's a source fence.
-			// Simplification: drop all bare ``` lines (no content on the line).
-			// Keep fences that are part of assistant markdown (they have text before/after).
-			if len(stripped) == 3 || (len(stripped) > 3 && stripped[3] != '`') {
-				inFence = true
-				continue
-			}
+		if bytes.HasPrefix(stripped, []byte("```")) && !bytes.HasPrefix(stripped, []byte("````")) {
+			continue
 		}
 
-		if !inFence {
-			out = append(out, line...)
-		}
+		out = append(out, line...)
 	}
 	return out
 }