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/advanced-text-editor/components/AdvancedTextEditor.tsx

@@ -30,6 +30,7 @@ const StyledEditorContainer = styled.div<{
     color: ${({ theme, readonly }) =>
       readonly ? theme.font.color.light : theme.font.color.primary};
     font-family: ${({ theme }) => theme.font.family};
+    font-size: ${({ theme }) => theme.font.size.sm};
     font-weight: ${({ theme }) => theme.font.weight.regular};
     border: none !important;
 
@@ -54,15 +55,15 @@ const StyledEditorContainer = styled.div<{
     }
 
     h1 {
-      font-size: 32px;
+      font-size: 1.5em;
     }
 
     h2 {
-      font-size: 24px;
+      font-size: 1.3em;
     }
 
     h3 {
-      font-size: 16px;
+      font-size: 1.1em;
     }
 
     li {