Add existence check to create-agent-session codemod

[Copilot]

The agent-task-to-agent-session-migration codemod migrates safe-outputs.create-agent-task to safe-outputs.create-agent-session but lacked a safety check for the target field's existence, risking data loss in workflows containing both fields.

Changes

• Safety check: Skip migration if create-agent-session already exists in frontmatter
• Logging: Log skip reason when target field exists
• Test coverage: Verify migration is skipped when both fields present

Behavior

Before:

safe-outputs:
  create-agent-task:
    title: Old Task
  create-agent-session:
    title: New Session  # Would be overwritten

After:

safe-outputs:
  create-agent-task:
    title: Old Task
  create-agent-session:
    title: New Session  # Preserved, migration skipped

The codemod now correctly implements the "if it does not exist yet" requirement by checking target field existence before transformation.

Original prompt

Create a codemox that renames "safe-outputs.create-agent-task" to "safe-outputs.create-agent-session" (if it does not exist yet)

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