Fix N+1 query when accessing topic.first_post in AI transitions

[Copilot]

Addresses performance feedback from PR #9: topic.first_post access on line 45 triggers N+1 queries since the association isn't preloaded.

Changes

• Changed includes from :topic to topic: :first_post to eagerly load the first post association

# Before
.includes(:topic, workflow_step: { workflow_step_options: :workflow_option })

# After  
.includes(topic: :first_post, workflow_step: { workflow_step_options: :workflow_option })

This eliminates one database query per workflow state when topic.first_post.raw is accessed during AI prompt construction.

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