Fix Repo Assist workflow: switch to copilot-requests auth and add checkout config

[Copilot]

Repo Assist has failed on every run since it was added — the workflow requires a COPILOT_GITHUB_TOKEN secret that was never configured in this repository.

Changes

• Enable copilot-requests feature flag — uses the built-in GitHub Actions token (${{ github.token }}) for Copilot authentication, eliminating the need for a separate secret
• Add checkout configuration — fetch: ["*"] + fetch-depth: 0 so the agent can access all PR branches, required by push-to-pull-request-branch: target: "*"
• Recompile lock file — gh aw compile --strict passes with 0 errors, 0 warnings

features:
  copilot-requests: true

checkout:
  fetch: ["*"]
  fetch-depth: 0

Key compiled lock file changes: COPILOT_GITHUB_TOKEN switches from ${{ secrets.COPILOT_GITHUB_TOKEN }} → ${{ github.token }}, copilot-requests: write permission added, secret validation step removed, S2STOKENS: true env injected.

[shanselman]

Thanks for putting together the original Repo Assist auth fix. This work directly informed the current upgrade path, but PR #33 now supersedes it with the newer gh-aw layout and v0.77.5 compiled workflow changes, so I’m closing this one in favor of #33.