feat(workitems): add list command
Description
This MR implements the `glab work-items list` command, the first in a series of commands for managing GitLab Work Items via the GraphQL API. The command allows users to query and display work items (epics, issues, tasks, incidents, etc.) from both projects and groups.
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Related Issues
Resolves #8113 (closed)
How has this been tested?
Unit Tests
- 24 unit tests with 100% coverage for utility functions
- Tests cover type validation, scope detection, and display formatting
- All tests passing: `go test ./internal/commands/workitems/... -v`
Manual Tests in CLI
Test Environment
- GitLab instance: gitlab.com
- Test groups/projects with nested paths (up to 5 levels deep)
- Multiple work item types: Epic, Issue, Task, Incident