feat: Introduce `azdo boards work-item` command group

[tmeckel]

This issue tracks the introduction of the azdo boards work-item command group. It bridges the top-level boards command with individual work item operations so that related commands share a consistent help surface and shared option parsing.

Command Group Description

Create a work-item subgroup beneath azdo boards that centralises work item management flows (listing, showing, creating, updating, etc.). The group should describe its relationship to Azure Boards work items and provide examples of typical scenarios (e.g., browsing the backlog, reviewing assignments, inspecting details).

Implementation Notes

• Add internal/cmd/boards/workitem/workitem.go that exposes NewCmd(ctx util.CmdContext) *cobra.Command.
• Register the new subgroup inside internal/cmd/boards/boards.go so that commands such as azdo boards work-item list are discoverable via the CLI hierarchy.
• Keep the short description focused on Azure Boards work items and ensure examples reference existing/planned leaf commands.
• Regenerate CLI docs (make docs) after wiring the subgroup so its help text appears in the published Markdown.
• Future work item subcommands should be registered from this package to avoid scattering wiring logic.

Command Wiring

• Implement the work-item group in internal/cmd/boards/workitem/workitem.go and ensure NewCmd only wires child commands under this package.
• Update internal/cmd/boards/boards.go to call cmd.AddCommand(workitem.NewCmd(ctx)) so the boards hierarchy exposes the work-item subtree.
• Run make docs once wiring is complete to update generated documentation.

SDK / Client Requirements

• Work-item leaf commands typically rely on the Work Item Tracking client and, depending on filters, the Identity and Extensions clients. Confirm that downstream commands call the existing helpers (ClientFactory().WorkItemTracking(...), ClientFactory().Identity(...), ClientFactory().Extensions(...)).
• If a required Azure DevOps client is missing from the factory, follow the "Handling Missing Azure DevOps SDK Clients" steps in AGENTS.md (extend ClientFactory, run go mod tidy and go mod vendor, update scripts/generate_mocks.sh, and implement the factory method) before adding command logic.

Tooling & Validation

• Format new Go code with gofmt/goimports.
• Run go build ./cmd/azdo/... to ensure the CLI compiles with the new wiring.
• Add/update hermetic tests and execute go test ./... (include documentation regeneration in the validation checklist).

Sub-Issues

• Implement azdo boards work-item list command (feat: Implement azdo boards work-item list command #136)

References

• Azure Boards work items overview
• WIQL documentation