feat: improve AI chat - system prompt, tool output, context window display (#17769)

⚠️ **AI-generated PR — not ready for review** ⚠️

cc @FelixMalfait

---

## Changes

### System prompt improvements
- Explicit skill-before-tools workflow to prevent the model from calling
tools without loading the matching skill first
- Data efficiency guidance (default small limits, use filters)
- Pluralized `load_skill` → `load_skills` for consistency with
`load_tools`

### Token usage reduction
- Output serialization layer: strips null/undefined/empty values from
tool results
- Lowered default `find_*` limit from 100 → 10, max from 1000 → 100

### System object tool generation
- System objects (calendar events, messages, etc.) now generate AI tools
- Only workflow-related and favorite-related objects are excluded

### Context window display fix
- **Bug**: UI compared cumulative tokens (sum of all turns) against
single-request context window → showed 100% after a few turns
- **Fix**: Track `conversationSize` (last step's `inputTokens`) which
represents the actual conversation history size sent to the model
- New `conversationSize` column on thread entity with migration

### Workspace AI instructions
- Support for custom workspace-level AI instructions

---------

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>

packages/twenty-front/src/modules/ai/components/AIChatThreadGroup.tsx

@@ -84,17 +84,18 @@ export const AIChatThreadGroup = ({
   const handleThreadClick = (thread: AgentChatThread) => {
     setCurrentAIChatThread(thread.id);
 
-    const totalTokens = thread.totalInputTokens + thread.totalOutputTokens;
     const hasUsageData =
-      totalTokens > 0 && isDefined(thread.contextWindowTokens);
+      (thread.conversationSize ?? 0) > 0 &&
+      isDefined(thread.contextWindowTokens);
 
     setAgentChatUsage(
       hasUsageData
         ? {
+            lastMessage: null,
+            conversationSize: thread.conversationSize ?? 0,
+            contextWindowTokens: thread.contextWindowTokens ?? 0,
             inputTokens: thread.totalInputTokens,
             outputTokens: thread.totalOutputTokens,
-            totalTokens,
-            contextWindowTokens: thread.contextWindowTokens ?? 0,
             inputCredits: thread.totalInputCredits,
             outputCredits: thread.totalOutputCredits,
           }