fix for https://github.com/antinomyhq/forgecode/issues/2846

[jeffbryner]

Fixes #2846 by respecting provider/model choice for agents.

Setting provider/model in an agent.md file now allows for things like using gemini pro vs flash for coding vs planning agents:

id: "coder"
title: "Write some serious code"
description: "Hands-on implementation agent .."
reasoning:
  enabled: true
provider: vertex_ai
model: gemini-3-flash-preview

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[jeffbryner]

Let me know if there is anything I can do to help merge, doesn't look like I can assist in any of the pending checks.