Introduce `azdo boards work-item relation` command group

[tmeckel]

Sub-issue of #138. Hardened spec — do not re-derive decisions. This umbrella introduces the azdo boards work-item relation subgroup beneath azdo boards work-item. Sibling umbrella: #138 itself.

Command Group Description

Create a relation subgroup beneath azdo boards work-item that centralises work-item link management flows. The group exposes the four Azure Boards work-item relation operations that map directly to the official AzDO Extension's boards work-item relation group (Python implementation at azure-devops/azext_devops/dev/boards/relations.py):

• azdo boards work-item relation add — attach a relation to a work item (Implement azdo boards work-item relation add command #272)
• azdo boards work-item relation list-type — list the relation types supported in the organization (Implement azdo boards work-item relation list-type command #273)
• azdo boards work-item relation remove — detach a relation from a work item (Implement azdo boards work-item relation remove command #274)
• azdo boards work-item relation show — list all relations of a work item (Implement azdo boards work-item relation show command #275)

Each leaf is a thin JSON-Patch wrapper over workitemtracking.Client.UpdateWorkItem (for add and remove) and a thin read wrapper over workitemtracking.Client.GetWorkItem / GetRelationTypes (for show and list-type). The REST surface is Work Items - Update (REST 7.1) and Relation Types - Get (REST 7.1).

Implementation Notes

• Add internal/cmd/boards/workitem/relation/relation.go that exposes NewCmd(ctx util.CmdContext) *cobra.Command.
• Register the new subgroup inside internal/cmd/boards/workitem/workitem.go so that commands such as azdo boards work-item relation add are discoverable via the CLI hierarchy.
• Keep the short description focused on Azure Boards work-item relations; do not duplicate descriptions of the parent work-item group.
• The Python implementation calls the work item with expand='All' after every write so that the response carries the populated relations array (with friendly names). We mirror this in all add/remove/show leaves.
• Each leaf command lives in its own subdirectory under internal/cmd/boards/workitem/relation/<leaf>/<leaf>.go per AGENTS.md "New Subgroup Under Existing Group" guidance.
• The four leaves share a small helper at internal/cmd/boards/workitem/relation/shared/relation.go that resolves a friendly relation-type name to its ReferenceName (mirroring get_system_relation_name in the Python source) and a second helper that fills friendly names in a *WorkItem's relations array (mirroring fill_friendly_name_for_relations_in_work_item). The helpers are owned by the umbrella and consumed by the leaves.
• The add leaf additionally needs util.SplitAndTrimCSV (in internal/cmd/util/csv.go or a relation-shared CSV helper) to support both repeated-flag and comma-separated target lists. If the umbrella is implemented before util.SplitAndTrimCSV exists in the codebase, the add leaf's implementation must add it as a relation-shared helper at internal/cmd/boards/workitem/relation/shared/csv.go to keep the scope contained.

Command Wiring

• Implement the relation group in internal/cmd/boards/workitem/relation/relation.go and ensure NewCmd only wires child commands under this package.
• Update internal/cmd/boards/workitem/workitem.go to call cmd.AddCommand(relation.NewCmd(ctx)) so the work-item hierarchy exposes the relation subtree.
• Each leaf exports NewCmd(ctx util.CmdContext) *cobra.Command from its own subdirectory.
• Run make docs once wiring is complete to update generated documentation.

SDK / Client Requirements

• All four leaves rely on the Work Item Tracking client via ClientFactory().WorkItemTracking(...).
• The add and remove leaves additionally call GetWorkItem and GetRelationTypes for friendly-name resolution and target validation.
• No new client methods are required: UpdateWorkItem, GetWorkItem, GetRelationTypes are all in the vendored SDK (vendor/.../v7/workitemtracking/client.go:205, 137, 113).

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).
• Per AGENTS.md, do not regenerate mocks: internal/mocks/workitemtracking_client_mock.go already contains the GetWorkItem, GetRelationTypes, and UpdateWorkItem mocks (lines 849, 669, 1358).

Sub-Issues

• Introduce azdo boards work-item relation subgroup wiring
• Implement azdo boards work-item relation add command (Implement azdo boards work-item relation add command #272)
• Implement azdo boards work-item relation list-type command (Implement azdo boards work-item relation list-type command #273)
• Implement azdo boards work-item relation remove command (Implement azdo boards work-item relation remove command #274)
• Implement azdo boards work-item relation show command (Implement azdo boards work-item relation show command #275)

References

• AzDO Extension Python source: https://github.com/Azure/azure-devops-cli-extension/blob/master/azure-devops/azext_devops/dev/boards/relations.py
• AzDO Extension command wiring: https://github.com/Azure/azure-devops-cli-extension/blob/master/azure-devops/azext_devops/dev/boards/commands.py (lines 92-99)
• AzDO Extension argument definitions: https://github.com/Azure/azure-devops-cli-extension/blob/master/azure-devops/azext_devops/dev/boards/arguments.py (lines 65-77)
• AzDO Extension table transformers: https://github.com/Azure/azure-devops-cli-extension/blob/master/azure-devops/azext_devops/dev/boards/_format.py (transform_work_item_relations, transform_work_item_relation_type_table_output)
• AzDO MCP Server (TypeScript): the expand: relations parameter on the read tool is the only relation handling — no dedicated relation tool exists. Reference: https://github.com/microsoft/azure-devops-mcp/blob/main/src/tools/work-items.ts
• Azure CLI reference: https://learn.microsoft.com/en-us/cli/azure/boards/work-item/relation
• Azure DevOps REST API: Work Items - Update (PATCH, API version 7.1-preview.3), Relation Types - Get (GET, API version 7.1-preview.2)
• Parent umbrella: feat: Introduce azdo boards work-item command group #138.