refactor: move copilot tools to dedicated copilot.go

Extract AssignCopilotToIssue, RequestCopilotReview, AssignCodingAgentPrompt,
and all supporting types/helpers from issues.go and pullrequests.go into
copilot.go and copilot_test.go.

This follows the existing convention where each domain (actions, dependabot,
discussions, gists, etc.) has its own file, and keeps the copilot toolset
implementation cohesive in one place.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SamMorrowDrums