Fix copilot bot assignment in create-issue to use GitHub API

[Copilot]

Copilot is a bot and requires GitHub API calls for issue assignment. The previous implementation used gh issue edit --add-assignee @copilot which doesn't work for bots.

Changes

• pkg/workflow/create_issue.go: Updated buildCreateOutputIssueJob to detect copilot assignees and use gh api --method POST /repos/.../issues/$ISSUE_NUMBER/assignees with copilot[bot] instead of the gh CLI command
• Tests: Updated unit and integration tests to verify GitHub API usage for copilot assignees while preserving existing behavior for regular users

Pattern

Now matches the PR reviewer implementation where copilot is handled via GitHub API:

safe-outputs:
  create-issue:
    assignees: [user1, copilot, user2]  # Mixed assignees supported

Generates:

• gh api POST /repos/.../issues/$ISSUE_NUMBER/assignees -f 'assignees[]=copilot[bot]' for copilot
• JavaScript assignee script via actions/github-script for regular users

Both use the same token precedence: GH_AW_COPILOT_TOKEN || GH_AW_GITHUB_TOKEN

Original prompt

Review the handling of "copilot" as a reviewer in create_issue. Since copilot is a bit, it needs to be treated differently to assign the issue to copilot agent.

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