feat: `azdo pipelines variable` command group

[tmeckel]

This issue tracks creating the azdo pipelines variable subgroup for managing definition-scoped pipeline variables.

Command Group Description

The Azure CLI provides az pipelines variable commands for CRUD operations on variables stored in a specific classic pipeline definition (build definition). azdo should expose an ergonomic subgroup that handles subject resolution (pipeline ID vs name), secret prompting, and JSON/list output consistent with the rest of the CLI.

Command Surface

Child issues will implement the following leaf commands:

• azdo pipelines variable create
• azdo pipelines variable update
• azdo pipelines variable list
• azdo pipelines variable delete

Scope & Argument Pattern

• Require a positional [ORGANIZATION/]PROJECT argument across the subgroup to select the project. When the organization segment is omitted, fall back to the configured default organization; error if neither is available.
• Each leaf command must accept either --pipeline-id (integer) or --pipeline (case-insensitive definition name) to target the pipeline definition, mirroring the Azure CLI behavior while avoiding positional ambiguity.
• Secret handling flags (--secret, --prompt-value) should mirror the Azure CLI semantics but follow azdo naming conventions.

Implementation Outline

• Add internal/cmd/pipelines/variable/variable.go exposing NewCmd(ctx util.CmdContext) *cobra.Command and registering leaf commands from sibling packages.
• Wire the subgroup inside internal/cmd/pipelines/pipelines.go via pipelinesCmd.AddCommand(variable.NewCmd(ctx)).
• Provide help text summarizing the available operations and clarifying pipeline targeting options.
• Ensure JSON export flags are available when a command needs to emit structured output.

Testing

• The grouping command itself does not need dedicated tests; child issues will add unit/acceptance coverage for functional paths.

References

• Azure CLI pipeline variable implementation: pipeline_variables.py
• Command registration: commands.py#L192-L204