feat: update assign-to-agent to use REST API instead of GraphQL

[mrjf]

Summary

Updates the assign-to-agent safe output to use the new REST API for assigning the Copilot coding agent to issues, as announced in the GitHub December 2025 changelog.

Key Changes

JavaScript Implementation

• assign_agent_helpers.cjs: Added new assignAgentToIssueRest() function using github.rest.issues.addAssignees() REST endpoint
• assign_agent_helpers.cjs: Added isAgentAssigned() to check assignment status via REST API
• assign_agent_helpers.cjs: Updated assignAgentToIssueByName() to try REST API first with GraphQL fallback
• assign_to_agent.cjs: Simplified to use REST API directly, removed GraphQL-specific code
• GraphQL methods marked as deprecated but kept for backwards compatibility

Go Changes

• assign_to_agent.go: Reduced permissions from NewPermissionsActionsWriteContentsWriteIssuesWritePRWrite() to NewPermissionsContentsReadIssuesWrite()

Documentation

• Updated safe-outputs.md with REST API details and simplified token requirements
• Updated test workflow with new permissions and documentation

Tests

• Added 9 new tests for assignAgentToIssueRest() function
• Added 5 new tests for isAgentAssigned() function
• Updated existing tests for REST API flow
• All 37 assign agent tests pass

Benefits

Before (GraphQL)	After (REST API)
4 write permissions required	Only issues: write needed
Multiple GraphQL queries	Single REST API call
Complex agent ID lookup	Direct login name usage
Limited error context	HTTP status-specific errors

REST API Endpoint Used

POST /repos/{owner}/{repo}/issues/{issue_number}/assignees
{
  "assignees": ["copilot-swe-agent"]
}

Reference

• GitHub Changelog: Assign issues to Copilot using the API

[mrjf]

@copilot Please find out why this dev.md run failed: https://github.com/githubnext/gh-aw/actions/runs/20039762441

[Copilot]

@mrjf I've opened a new pull request, #5899, to work on those changes. Once the pull request is ready, I'll request review from you.

[mrjf]

What! I didn't merge this!