wip: context-meter + model-label UI and provider/inference tweaks

Checkpoint of in-flight work so the orchestrator branch can rebase onto a
clean main: ContextBar → ContextMeter, model-label helper, model/agent picker
+ provider-snapshot/registry changes, inference payload + message-columns.

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

apps/server/src/services/inference/payload.ts

@@ -91,6 +91,27 @@ export async function buildMessagesPayload(
     }
   }
 
+  // Per-turn model attribution. When the sent window mixes ≥2 models, prefix
+  // each prior assistant turn with its model id so the active model can answer
+  // "what did Opus say". Single-model chats are left byte-identical (no prefix,
+  // no note) so the common case sees no payload or prefix-cache change.
+  const sentModels = new Set<string>();
+  for (let i = startIdx; i < history.length; i++) {
+    const m = history[i]!;
+    if (m.role === 'assistant' && m.model && !isAnySentinel(m)) sentModels.add(m.model);
+  }
+  const annotateModels = sentModels.size >= 2;
+  if (annotateModels) {
+    out.push({
+      role: 'system',
+      content:
+        'This conversation includes replies from more than one AI model. Each prior ' +
+        'assistant turn below is prefixed with its model id in square brackets, e.g. ' +
+        '[claude-opus-4-8]. Those prefixes are metadata for your reference (so you can ' +
+        'tell which model produced which turn) — do not add such a prefix to your own replies.',
+    });
+  }
+
   for (let i = startIdx; i < history.length; i++) {
     const m = history[i]!;
     if (m.kind === 'compact') {
@@ -143,9 +164,10 @@ export async function buildMessagesPayload(
       continue;
     }
     if (m.role === 'assistant') {
+      const body = m.content && m.content.length > 0 ? m.content : null;
       const msg: OpenAiMessage = {
         role: 'assistant',
-        content: m.content && m.content.length > 0 ? m.content : null,
+        content: body != null && annotateModels && m.model ? `[${m.model}] ${body}` : body,
       };
       if (m.tool_calls && m.tool_calls.length > 0) {
         if (assistantToolCallsArePayloadComplete(history, i)) {