refactor(issues): migrate issues.go to NewTool pattern

Convert all 8 tool functions in issues.go to use the new NewTool
helper pattern which standardizes dependency injection:

- IssueRead: GetClient, GetGQLClient, RepoAccessCache, Flags
- ListIssueTypes: GetClient
- AddIssueComment: GetClient
- SubIssueWrite: GetClient
- SearchIssues: GetClient
- IssueWrite: GetClient, GetGQLClient
- ListIssues: GetGQLClient
- AssignCopilotToIssue: GetGQLClient

Updated tools.go to use direct function calls instead of
NewServerToolLegacy wrappers. Updated all tests in issues_test.go
to use the new ToolDependencies pattern and Handler() method.

Co-authored-by: Adam Holt <omgitsads@users.noreply.github.com>

Author: SamMorrowDrums