Replace agent handoff system with planning-based router (#16003)

## Overview

This PR replaces the dynamic agent handoff system with a more
predictable planning-based router that decides upfront how to handle
multi-agent coordination.

## Major Changes

### 🔄 Architecture Shift: Handoffs → Planning

**Removed:**
- `AgentHandoffEntity` and handoff tracking system
- `AgentHandoffService` and `AgentHandoffExecutorService`
- Dynamic agent-to-agent transfers during execution
- Handoff tool generation and description templates

**Added:**
- `AiRouterService` with two strategies: `simple` (single agent) and
`planned` (multi-agent)
- `AgentPlanExecutorService` for executing multi-step plans
- Plan validation (cycle detection, dependency resolution)
- `UnifiedRouterResult` type with discriminated union

### 🤖 New Standard Agents

Added two new specialized agents:
- **Researcher Agent**: Web search, fact-finding, competitive
intelligence
- **Code Agent**: TypeScript function generation for serverless
workflows

### 🏗️ Router Refactoring (Latest)

Split router responsibilities into focused services:
- `AiRouterStrategyDeciderService`: Decides simple vs planned strategy
- `AiRouterPlanGeneratorService`: Generates and validates execution
plans
- `AiRouterService`: Coordinates between services (reduced from 426→275
lines)

### ⚙️ Configuration Improvements

- Added `outputStrategy` to agent definitions (`direct` vs `synthesize`)
- Removed hardcoded special cases for workflow-builder
- Added `plannerModel` field to workspace entity
- Increased `MAX_STEPS` from 10 to 25 for complex workflows

### 📝 Agent Prompt Refinements

Significantly simplified prompts for better clarity:
- Workflow Builder: 51→36 lines
- Helper: 49→28 lines
- Data Manipulator: Enhanced with sorting guidance

### 🔍 Enhanced Debugging

- Plan reasoning and step count in data message parts
- Router debug info with token usage tracking
- Better logging throughout execution pipeline

## Benefits

1. **Simpler Mental Model**: Router decides upfront vs dynamic transfers
2. **Better Predictability**: Users see the plan before execution
3. **Cleaner Architecture**: SRP with focused services
4. **Configuration Over Code**: Agent behavior via config, not hardcoded
logic
5. **Plan Validation**: Catches invalid dependencies and cycles

## Migration Notes

- Database migration removes `agentHandoff` table
- Adds `plannerModel` column to workspace table
- No API breaking changes (agent endpoints unchanged)

## Testing

- Integration tests updated to remove handoff dependencies
- Agent tool test utilities simplified
- Plan validation covered by new logic

## Next Steps (Future PRs)

- Parallel execution of independent plan steps
- Dynamic re-planning based on results
- Plan caching for common routing patterns
- Error recovery strategies in plan executor

packages/twenty-server/src/engine/metadata-modules/metadata-engine.module.ts

@@ -1,6 +1,7 @@
 import { Module } from '@nestjs/common';
 
-import { AgentModule } from 'src/engine/metadata-modules/agent/agent.module';
+import { AiAgentModule } from 'src/engine/metadata-modules/ai-agent/ai-agent.module';
+import { AiChatModule } from 'src/engine/metadata-modules/ai-chat/ai-chat.module';
 import { CronTriggerModule } from 'src/engine/metadata-modules/cron-trigger/cron-trigger.module';
 import { DataSourceModule } from 'src/engine/metadata-modules/data-source/data-source.module';
 import { DatabaseEventTriggerModule } from 'src/engine/metadata-modules/database-event-trigger/database-event-trigger.module';
@@ -25,7 +26,8 @@ import { WorkspaceMigrationModule } from 'src/engine/metadata-modules/workspace-
     SearchFieldMetadataModule,
     ServerlessFunctionModule,
     ServerlessFunctionLayerModule,
-    AgentModule,
+    AiAgentModule,
+    AiChatModule,
     ViewModule,
     WorkspaceMetadataVersionModule,
     WorkspaceMigrationModule,
@@ -43,7 +45,8 @@ import { WorkspaceMigrationModule } from 'src/engine/metadata-modules/workspace-
     ObjectMetadataModule,
     SearchFieldMetadataModule,
     ServerlessFunctionModule,
-    AgentModule,
+    AiAgentModule,
+    AiChatModule,
     ViewModule,
     RemoteServerModule,
     RoleModule,