[FEATURE] Implement Workflow Subagents with ADK Patterns

[dimetron]

📋 Prerequisites

• I have searched the existing issues to avoid creating a duplicate
• By submitting this issue, you agree to follow our Code of Conduct

📝 Feature Summary

Declarative workflow patterns (Sequential, Parallel, Loop) from ADK's workflow agents

❓ Problem Statement / Motivation

This feature adds declarative workflow patterns (Sequential, Parallel, Loop) to kagent using ADK's workflow agents. Implementation follows Test-Driven Development with parallel execution where possible.

💡 Proposed Solution

ADK Workflow Example:

Reference: https://github.com/google/adk-docs/blob/main/docs/agents/workflow-agents/loop-agents.md

# Writer Agent
apiVersion: kagent.dev/v1alpha2
kind: Agent
metadata:
  name: document-writer
  namespace: kagent
spec:
  type: Declarative
  description: "Generates or refines document drafts"
  declarative:
    systemMessage: |
      You are a skilled technical writer. 
      
      First iteration: Generate an initial draft based on the user's topic.
      Subsequent iterations: Read previous draft (temp:document_draft) and 
      feedback (temp:critique), then improve the document.
      
      Focus on:
      - Clear structure
      - Accurate technical content
      - Engaging writing style
      - Proper formatting
      
      Store your draft in output_key: "document_draft"
    modelConfig: default-model-config
    tools:
      - type: McpServer
        mcpServer:
          name: web-search-mcp
          toolNames: ["search", "get_webpage_content"]

---
# Critic Agent
apiVersion: kagent.dev/v1alpha2
kind: Agent
metadata:
  name: document-critic
  namespace: kagent
spec:
  type: Declarative
  description: "Critiques documents and suggests improvements"
  declarative:
    systemMessage: |
      You are an experienced editor and critic.
      
      Read the current draft from temp:document_draft.
      
      Evaluate:
      - Structure and organization
      - Clarity and readability
      - Technical accuracy
      - Completeness
      - Grammar and style
      
      Provide constructive criticism with specific suggestions for improvement.
      If the document is excellent (no major issues), you may recommend stopping.
      
      Store your critique in output_key: "critique"
    modelConfig: default-model-config

---
# Iterative Document Improver with Loop
apiVersion: kagent.dev/v1alpha2
kind: Agent
metadata:
  name: iterative-document-improver
  namespace: kagent
spec:
  type: Declarative
  description: "Iteratively improves documents through write-critique-refine cycles"
  declarative:
    systemMessage: |
      You orchestrate an iterative document improvement process.
      
      The workflow loops through:
      1. Writer creates/refines the document
      2. Critic evaluates and provides feedback
      
      This continues for up to 5 iterations or until the document quality is satisfactory.
      
      After the loop completes, present the final polished document to the user.
    modelConfig: default-model-config
    
    subagents:
      - role: "Iterative Document Improver"
        description: "Loop workflow for document refinement"
        delegationMode: Supervised
        workflow:
          type: Loop
          agents:
            - name: document-writer
              description: "Writes or refines document drafts"
            - name: document-critic
              description: "Critiques and suggests improvements"
            # the critic agent use exit_loop to stop the loop
          loopConfig:
            maxIterations: 5

🔄 Alternatives Considered

No response

🎯 Affected Service(s)

None

📚 Additional Context

No response

🙋 Are you willing to contribute?

• I am willing to submit a PR for this feature