feat: Implement `azdo pipelines variable-group variable list` command

[tmeckel]

This issue tracks the implementation of the azdo pipelines variable-group variable list command.

Command Description

List the variables in a variable group. The Azure CLI retrieves the group and returns the variables dictionary as-is (source). azdo should expose the same information with user-friendly formatting, readOnly metadata, and JSON export.

azdo Command Signature

azdo pipelines variable-group variable list [ORGANIZATION/]PROJECT/VARIABLE_GROUP_ID_OR_NAME

Behavior

• Resolve project scope and group identifier via the shared helper.
• Start the progress indicator, fetch the group, then stop the indicator prior to rendering output.
• If the group is missing, return a descriptive error.
• Sort variables alphabetically for deterministic output.
• Default table columns: NAME, VALUE (mask when secret), SECRET (Yes/No), READ-ONLY (Yes/No).
• Register JSON fields with util.AddJSONFlags(cmd, &opts.exporter, []string{"name","secret","readOnly","value"}) and emit a slice of { name, secret, readOnly, value *string }. Drop the value pointer when secret is true so filtering via --json name,secret works predictably.
• Add zap.L().Debug logs for group resolution.

Command Wiring

• Implement internal/cmd/pipelines/variablegroup/variable/list/list.go with NewCmd(ctx util.CmdContext) *cobra.Command.
• Register it from internal/cmd/pipelines/variablegroup/variable/variable.go so the hierarchy exposes azdo pipelines variable-group variable list.
• Run make docs after wiring.

SDK / Client Requirements

• Requires the Task Agent client (ClientFactory().TaskAgent(...)). If missing, follow Handling Missing Azure DevOps SDK Clients.

Tooling & Validation

• gofmt/goimports new files.
• go build ./cmd/azdo/... to ensure compilation.
• Add/update hermetic tests with mocks; run go test ./....

Testing

1. Successful listing with secret + non-secret variables.
2. Missing group error.
3. Deterministic ordering.
4. JSON export structure.

References

• Azure CLI: variable_group_variable_list
• Azure DevOps REST API 7.1: Update variable group