feat: `azdo boards` command group

[tmeckel]

This issue introduces the top-level azdo boards command group so users can manage Azure Boards resources directly from azdo.

Command Group Description

Create a boards parent command that is registered under the root CLI and acts as the entry point for work item and backlog management scenarios. The group should provide concise help text describing its relationship to Azure Boards and should follow the repository's conventions for wiring new groups (directory structure, NewCmd(ctx) factory, documentation regeneration, etc.).

Implementation Notes

• Add a new package at internal/cmd/boards that defines NewCmd(ctx util.CmdContext) *cobra.Command and registers existing/future subcommands.
• Ensure the new command group is added to the root command (internal/cmd/root/root.go) and appears in generated docs (make docs).
• Maintain consistency with other top-level groups (aliases, short description, examples scaffold) to keep the CLI ergonomic.
• Delegate feature-specific wiring to dedicated subgroups (work items, area paths, iteration paths) so leaf commands live one level deeper.

Command Wiring

• Implement the group factory in internal/cmd/boards/boards.go and ensure it registers the child subgroups (work-item, area, iteration).
• Update internal/cmd/root/root.go to include rootCmd.AddCommand(boards.NewCmd(ctx)) so the CLI exposes the new top-level entry point.
• Run make docs after wiring changes to refresh generated help content.

SDK / Client Requirements

• The top-level group itself does not introduce new SDK dependencies. Subcommands under boards consume specific clients (Work Item Tracking, Identity, etc.).
• If adding new Azure DevOps clients becomes necessary for future boards subcommands, follow the "Handling Missing Azure DevOps SDK Clients" procedure in AGENTS.md (extend ClientFactory, run go mod tidy and go mod vendor, update scripts/generate_mocks.sh, and implement the factory method) before shipping command logic.

Tooling & Validation

• Format new Go files with gofmt/goimports.
• Run go build ./cmd/azdo/... to confirm the CLI compiles with the new group wiring.
• Add/update hermetic tests and execute go test ./...; regenerate documentation as part of the validation checklist.

Sub-Issues

• Introduce azdo boards work-item command group (feat: Introduce azdo boards work-item command group #138)
• Introduce azdo boards area command group (feat: Introduce azdo boards area command group #139)
• Introduce azdo boards iteration command group (feat: Introduce azdo boards iteration command group #140)

References

• Azure Boards documentation