fix(agent): preserve up to 30 media refs during history compaction

Both v3 mid-loop compaction and v2 background summarization were
dropping MediaRefs when summarizing old messages, making previously
shared images/documents permanently inaccessible to the agent.

Now collect up to 30 most recent MediaRefs from compacted messages
and attach them to the summary/first-kept message so they survive
the compaction cycle.

internal/agent/loop_compact.go

@@ -100,9 +100,22 @@ func (l *Loop) compactMessagesInPlace(ctx context.Context, messages []providers.
 		return nil
 	}
 
+	// Collect MediaRefs from compacted messages (keep up to 30 most recent).
+	const maxPreservedMediaRefs = 30
+	var preservedRefs []providers.MediaRef
+	for i := len(toSummarize) - 1; i >= 0 && len(preservedRefs) < maxPreservedMediaRefs; i-- {
+		for _, ref := range toSummarize[i].MediaRefs {
+			preservedRefs = append(preservedRefs, ref)
+			if len(preservedRefs) >= maxPreservedMediaRefs {
+				break
+			}
+		}
+	}
+
 	summary := providers.Message{
-		Role:    "user",
-		Content: "[Summary of earlier conversation]\n" + SanitizeAssistantContent(resp.Content),
+		Role:      "user",
+		Content:   "[Summary of earlier conversation]\n" + SanitizeAssistantContent(resp.Content),
+		MediaRefs: preservedRefs,
 	}
 	result := make([]providers.Message, 0, 1+keepCount)
 	result = append(result, summary)

internal/agent/loop_history_sanitize.go

@@ -292,8 +292,33 @@ func (l *Loop) maybeSummarize(ctx context.Context, sessionKey string) {
 			return
 		}
 
+		// Collect MediaRefs from messages about to be truncated (keep up to 30 most recent).
+		const maxPreservedMediaRefs = 30
+		var preservedRefs []providers.MediaRef
+		for i := len(toSummarize) - 1; i >= 0 && len(preservedRefs) < maxPreservedMediaRefs; i-- {
+			for _, ref := range toSummarize[i].MediaRefs {
+				preservedRefs = append(preservedRefs, ref)
+				if len(preservedRefs) >= maxPreservedMediaRefs {
+					break
+				}
+			}
+		}
+
 		l.sessions.SetSummary(sctx, sessionKey, SanitizeAssistantContent(resp.Content))
 		l.sessions.TruncateHistory(sctx, sessionKey, keepLast)
+
+		// Inject preserved MediaRefs into the first kept message so they survive truncation.
+		if len(preservedRefs) > 0 {
+			kept := l.sessions.GetHistory(sctx, sessionKey)
+			if len(kept) > 0 {
+				kept[0].MediaRefs = append(preservedRefs, kept[0].MediaRefs...)
+				// Cap total refs on this message at maxPreservedMediaRefs.
+				if len(kept[0].MediaRefs) > maxPreservedMediaRefs {
+					kept[0].MediaRefs = kept[0].MediaRefs[:maxPreservedMediaRefs]
+				}
+				l.sessions.SetHistory(sctx, sessionKey, kept)
+			}
+		}
 		l.sessions.IncrementCompaction(sctx, sessionKey)
 		// Mirror SessionMetaKeyLastCompactionAt from the v3 prune/compact path
 		// so the legacy v2 post-turn summarizer also surfaces compaction cadence.