attribute sidechain prose/thinking to the agent, not claude (T-265); close T-267

A sub-agent's prose rendered as "claude" and its thinking as "thinking",
identical to the main assistant, because the label logic ignored
isSidechain — presenting sub-agent output as if the main Claude said it.
Now a sidechain AssistantTextMessage is labelled "agent" with a muted
stripe (never the coral claudeAccent brand), and sidechain thinking is
"agent thinking". Main-thread items are unchanged.

Tests: sidechain prose/thinking relabel, main-thread unchanged; golden
contrasting the muted agent stripe with the coral claude stripe.

This completes epic T-267 (conversation rendering streamlining): fold
success result (T-262), fold agent prompt (T-263), nest the agent run
(T-264), the shared holder primitive (T-266), and this attribution fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/claude/src/conversation_view.dart

@@ -354,16 +354,19 @@ class _ConversationTurn extends StatelessWidget {
           copyText: i.text,
           body: ClideMarkdown(i.text),
         ),
+      // Sub-agent (sidechain) prose is NOT the main Claude — attribute it to the
+      // agent with a muted accent, never the coral "claude" brand (T-265). The
+      // coral claudeAccent is reserved for the real main-thread Claude.
       AssistantTextMessage() => ConversationCard(
-          accent: claudeAccent,
-          label: 'claude',
+          accent: i.isSidechain ? tokens.globalTextMuted : claudeAccent,
+          label: i.isSidechain ? 'agent' : 'claude',
           copyText: i.text,
           body: ClideMarkdown(i.text),
         ),
       AssistantThinkingMessage() => ConversationCard(
           variant: ConversationCardVariant.bare,
           accent: tokens.globalTextMuted,
-          label: 'thinking',
+          label: i.isSidechain ? 'agent thinking' : 'thinking',
           copyText: i.thinking,
           collapsible: true,
           collapsedByDefault: true,