Refactor ModifyAgent mutation to use adapter pattern

## Objective

Simplify the ModifyAgent GraphQL mutation resolver by applying the adapter pattern, making it a thin layer that delegates to the service layer.

## Current Issues

- GraphQL resolver contains business logic
- Direct coupling to registry and database operations
- Difficult to test in isolation

## Implementation Scope

- Refactor ModifyAgent.mutate() to use ActionProcessor pattern
- Transform GraphQL input (ModifyAgentInput) to service Action
- Transform service ActionResult to GraphQL response
- Delegate business logic to AgentService (from BA-3571)

## Dependencies

- Requires BA-3571 to be completed first

## Expected Outcome

ModifyAgent resolver becomes a thin adapter layer, following the same pattern as other modern GraphQL mutations in the codebase.

JIRA Issue: BA-3572