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-server/src/database/typeorm/core/migrations/common/1770311652940-add-ai-additional-instructions.ts

@@ -0,0 +1,19 @@
+import { type MigrationInterface, type QueryRunner } from 'typeorm';
+
+export class AddAiAdditionalInstructions1770311652940
+  implements MigrationInterface
+{
+  name = 'AddAiAdditionalInstructions1770311652940';
+
+  public async up(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.query(
+      `ALTER TABLE "core"."workspace" ADD "aiAdditionalInstructions" text`,
+    );
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.query(
+      `ALTER TABLE "core"."workspace" DROP COLUMN "aiAdditionalInstructions"`,
+    );
+  }
+}

packages/twenty-server/src/database/typeorm/core/migrations/common/1770400000000-addConversationSizeToAgentChatThread.ts

@@ -0,0 +1,19 @@
+import { type MigrationInterface, type QueryRunner } from 'typeorm';
+
+export class AddConversationSizeToAgentChatThread1770400000000
+  implements MigrationInterface
+{
+  name = 'AddConversationSizeToAgentChatThread1770400000000';
+
+  public async up(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.query(
+      `ALTER TABLE "core"."agentChatThread" ADD COLUMN "conversationSize" integer NOT NULL DEFAULT 0`,
+    );
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.query(
+      `ALTER TABLE "core"."agentChatThread" DROP COLUMN "conversationSize"`,
+    );
+  }
+}