fix: allow opted-in non-GPT Hephaestus registration

[dyyxxhh]

Summary

• Lets agents.hephaestus.allow_non_gpt_model: true bypass Hephaestus registration-time model checks.
• Passes the opt-in through to Hephaestus prompt construction, where unsupported models use the generic GPT prompt instead of throwing.
• Keeps the default behavior unchanged: without the explicit opt-in, non-GPT Hephaestus models are still skipped/rejected.

Test Plan

• npx --yes bun@1.3.14 test packages/omo-opencode/src/agents/hephaestus/agent.test.ts
• npx --yes bun@1.3.14 run typecheck:packages

Related question: #5399

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Summary by cubic

Allow Hephaestus to register with non-GPT models when opted in via agents.hephaestus.allow_non_gpt_model: true, falling back to the generic GPT prompt instead of rejecting. Default behavior stays the same; without the flag, unsupported models are skipped.

• Bug Fixes
  • Gate model support checks behind the opt-in during registration.
  • Plumb allowUnsupportedModel to prompt construction; use GPT prompt for unsupported models.
  • Add test to verify non-GPT registration with the flag and preserve apply_patch permission.

^{Written for commit 954afda. Summary will update on new commits.}

[EvangelosMoschou]

Confirmed the approach matches what I was implementing locally. The fallback to the generic GPT prompt in agent.ts is clean, and the AgentOverrides type correctly exposes allow_non_gpt_model for hephaestus — no type cast needed. Thanks for taking this on.