Introduce `azdo pipelines build tag` command group

[tmeckel]

Introduce the azdo pipelines build tag subgroup beneath azdo pipelines build (#213). This umbrella tracks the wiring of the subgroup and lists the three leaf commands that operate on build tags.

Command Group Description

Create a tag subgroup beneath azdo pipelines build that centralises tag-management flows for a single Azure Pipelines build. The group exposes the three Azure Pipelines build-tag operations that map directly to the official AzDO Extension's pipelines build tag group (Python implementation at azure-devops/azext_devops/dev/pipelines/build.py):

• azdo pipelines build tag add — attach one or more tags to a build (feat: Implement azdo pipelines build tag add command #277)
• azdo pipelines build tag delete — remove a tag from a build (feat: Implement azdo pipelines build tag delete command #278)
• azdo pipelines build tag list — list the tags attached to a build (feat: Implement azdo pipelines build tag list command #279)

Each leaf is a thin wrapper over the vendored build.Client (singular AddBuildTag, bulk AddBuildTags, DeleteBuildTag, GetBuildTags). The REST surface is Builds - Tags (REST 7.1-preview.3).

> Note: this group is distinct from the pipelines build definition subgroup (planned but not yet filed) which manages tags on a build definition (uses AddDefinitionTag / DeleteDefinitionTag). Tags on a build record are immutable once recorded; tags on a definition are persisted and can be inherited by builds.

Implementation Notes

• Add internal/cmd/pipelines/build/tag/tag.go that exposes NewCmd(ctx util.CmdContext) *cobra.Command.
• Register the new subgroup inside internal/cmd/pipelines/build/build.go so that commands such as azdo pipelines build tag list are discoverable via the CLI hierarchy. The existing umbrella feat: Introduce azdo pipelines build command group #213 already lists list, show, cancel, queue as children of build; this umbrella appends tag as a new child group.
• Keep the short description focused on per-build tag management; do not duplicate descriptions of the parent build group.
• The Python implementation only accepts a single positional build ID; in our Go port the build ID lives in the third segment of the [ORG:]PROJECT/BUILD positional, mirroring cancel (feat: Implement azdo pipelines build cancel command #252) and queue (feat: Implement azdo pipelines build queue command #253).
• Each leaf command lives in its own subdirectory under internal/cmd/pipelines/build/tag//.go per AGENTS.md "New Subgroup Under Existing Group" guidance.
• No new SDK clients are required: AddBuildTag, AddBuildTags, DeleteBuildTag, and GetBuildTags are all in the vendored SDK at vendor/.../v7/build/client.go:29, 31, 51, 111. Mocks are already generated at internal/mocks/build_client_mock.go:46, 61, 210.

Command Wiring

• Implement the relation group in internal/cmd/pipelines/build/tag/tag.go and ensure NewCmd only wires child commands under this package.
• Update internal/cmd/pipelines/build/build.go to call cmd.AddCommand(tag.NewCmd(ctx)) so the build hierarchy exposes the tag 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 three leaves rely on the Build client via ClientFactory().Build(...).
• No new client methods are required.
• No new mocks are required (all four are already generated).

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

• Introduce azdo pipelines build tag subgroup wiring
• Implement azdo pipelines build tag add command (feat: Implement azdo pipelines build tag add command #277)
• Implement azdo pipelines build tag delete command (feat: Implement azdo pipelines build tag delete command #278)
• Implement azdo pipelines build tag list command (feat: Implement azdo pipelines build tag list command #279)

References

• Umbrella: feat: Introduce azdo pipelines build command group #213 (azdo pipelines build command group).
• Sibling subgroups: feat: Introduce azdo pipelines runs command group #214 (azdo pipelines runs), Introduce azdo pipelines folder command group #262 (azdo pipelines folder).
• AzDO Extension Python source: https://github.com/Azure/azure-devops-cli-extension/blob/master/azure-devops/azext_devops/dev/pipelines/build.py — add_build_tags (line 184), delete_build_tag (line 198), get_build_tags (line 210).
• AzDO Extension table transformer: https://github.com/Azure/azure-devops-cli-extension/blob/master/azure-devops/azext_devops/dev/pipelines/_format.py — transform_build_tags_output (single-column Tags table).
• AzDO Extension command registration: https://github.com/Azure/azure-devops-cli-extension/blob/master/azure-devops/azext_devops/dev/pipelines/commands.py — g.command('add', 'add_build_tags', table_transformer=transform_build_tags_output) and g.command('delete', 'delete_build_tag', table_transformer=transform_build_tags_output).
• AzDO MCP Server (TypeScript): no dedicated build tag tools. The only tag reference in pipelines.ts is the tagFilters parameter on the list-runs tool. Reference: https://github.com/microsoft/azure-devops-mcp/blob/main/src/tools/pipelines.ts
• Azure CLI reference: https://learn.microsoft.com/en-us/cli/azure/pipelines/build/tag
• Azure DevOps REST API: Build Tags - Get (REST 7.1-preview.3) (also covers add/delete via the same 6e6114b2-8161-44c8-8f6c-c5505782427f location ID).