fix: use gpt-5-mini model for repo-assist workflow

[popey]

Summary

• Replaces claude-sonnet-4.6 (unsupported by this Copilot subscription tier) with gpt-5-mini as the fallback model in the repo-assist workflow
• Updates all three occurrences in repo-assist.lock.yml (GH_AW_INFO_MODEL + both COPILOT_MODEL env vars)
• Adds engine: copilot and model: gpt-5-mini to the repo-assist.md source frontmatter

Closes #41

Test plan

• Merge and confirm the next scheduled repo-assist run completes without a "Model Not Supported" error

🤖 Generated with Claude Code

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Summary by cubic

Switches the repo-assist workflow to gpt-5-mini as the Copilot model, replacing unsupported claude-sonnet-4.6 to stop “Model Not Supported” failures. Aligns the workflow with our Copilot plan and addresses #41.

• Bug Fixes
  • Set fallback model to gpt-5-mini in .github/workflows/repo-assist.lock.yml for GH_AW_INFO_MODEL and both COPILOT_MODEL env vars.
  • Added engine: copilot and model: gpt-5-mini to .github/workflows/repo-assist.md frontmatter.

^{Written for commit f45b0e5. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}