Add GraphQL-Features header for Copilot assignment mutations

[Copilot]

GitHub requires the GraphQL-Features: issues_copilot_assignment_api_support header for Copilot assignment mutations since Dec 2025. The framework was generating these mutations without it, causing assignment failures.

Changes

• Added required header to replaceActorsForAssignable mutation (primary path)
• Added required header to addAssigneesToAssignable mutation (fallback path)

Both mutations in actions/setup/js/assign_agent_helpers.cjs now pass headers via the options object:

await github.graphql(mutation, {
  assignableId: assignableId,
  actorIds,
  headers: {
    "GraphQL-Features": "issues_copilot_assignment_api_support",
  },
});

This eliminates the need for post-compilation lock file patches.

Original prompt

Review this error report and apply fix:

The gh-aw framework generates the replaceActorsForAssignable mutation code internally, and it's missing the GraphQL-Features:
issues_copilot_assignment_api_support header that GitHub requires since Dec 2025.

What to tell others:

• The gh-aw framework has a bug: its assign-to-agent safe output doesn't include the required GraphQL feature flag header for Copilot assignment
• Our fix patches the lock.yml to use the fallback mutation (addAssigneesToAssignable) which works with the header
• After any gh aw compile, run .github/aw/patch-lock-files.sh to re-apply the patch
• This should be reported to the gh-aw team for a proper fix

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[clubanderson]

lgtm