feat: Introduce `azdo pipelines variable-group` subgroup

[tmeckel]

This issue tracks establishing the azdo pipelines variable-group subgroup under the top-level pipelines hierarchy.

Command Group Description

Variable groups are reusable name/value collections that can be shared across multiple pipelines. The Azure CLI exposes them under az pipelines variable-group with CRUD operations and nested commands for managing individual variables. In azdo, this subgroup should own all variable-group scoped commands and provide consistent argument parsing, progress indicators, JSON output, and table formatting per repository conventions.

Command Surface

Follow-up issues cover each leaf command; this parent wires the subgroup and shared helpers:

• azdo pipelines variable-group create (feat: Implement azdo pipelines variable-group create command #119)
• azdo pipelines variable-group show (feat: Implement azdo pipelines variable-group show command #120)
• azdo pipelines variable-group list (feat: Implement azdo pipelines variable-group list command #121)
• azdo pipelines variable-group update (feat: Implement azdo pipelines variable-group update command #122)
• azdo pipelines variable-group delete (feat: Implement azdo pipelines variable-group delete command #123)
• azdo pipelines variable-group variable create (feat: Implement azdo pipelines variable-group variable create command #124)
• azdo pipelines variable-group variable list (feat: Implement azdo pipelines variable-group variable list command #125)
• azdo pipelines variable-group variable update (feat: Implement azdo pipelines variable-group variable update command #126)
• azdo pipelines variable-group variable delete (feat: Implement azdo pipelines variable-group variable delete command #127)

Scope & Argument Pattern

• All child commands share the [ORGANIZATION/]PROJECT positional argument pattern from ISSUE-TRIAGE.md. When the organization segment is omitted, fall back to the configured default; otherwise validate the provided organization URL.
• Provide persistent JSON flags via util.AddJSONFlags(cmd, &opts.exporter, ...) so leaf commands can reuse the configured exporter.
• Enforce kebab-case flags within the subgroup (e.g., --group-id, --authorize, --variable).

Implementation Outline

• Create internal/cmd/pipelines/variablegroup/variablegroup.go defining NewCmd(ctx util.CmdContext) *cobra.Command with Use: "variable-group", descriptions, and child registration hooks (cmd.AddCommand(create.NewCmd(ctx)), etc.).
• Ensure the command only resolves IO streams when necessary and keeps shared helpers (scope parsing, variable-group resolution, authorization toggles) in dedicated internal packages for reuse.
• Wire the subgroup inside internal/cmd/pipelines/pipelines.go via pipelinesCmd.AddCommand(variablegroup.NewCmd(ctx)).
• Provide help examples covering project-scoped usage and JSON export.
• Run make docs after the subgroup and leaf commands land so generated markdown reflects the hierarchy.

Testing

• The grouping command does not need direct unit tests, but ensure acceptance/smoke tests that rely on command discovery are updated after wiring.

References

• Azure CLI implementation for variable groups: variable_group.py
• Command registration in Azure CLI: commands.py#L150-L196