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

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

packages/twenty-front/src/modules/auth/states/currentWorkspaceState.ts

@@ -36,6 +36,7 @@ export type CurrentWorkspace = Pick<
   | 'eventLogRetentionDays'
   | 'fastModel'
   | 'smartModel'
+  | 'aiAdditionalInstructions'
   | 'editableProfileFields'
 > & {
   defaultRole?: Omit<Role, 'workspaceMembers' | 'agents' | 'apiKeys'> | null;