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

[tmeckel]

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

Command Description

Delete a variable group from a project. The Azure CLI issues TaskAgentClient.delete_variable_group and prints a success message (source). azdo should provide the same capability with confirmation prompts, consistent output handling, and the shared resolver used by the other variable-group commands.

azdo Command Signature

azdo pipelines variable-group delete [ORGANIZATION/]PROJECT/VARIABLE_GROUP_ID_OR_NAME [--yes]

Flags:

• --yes, -y: Skip the confirmation prompt.
• JSON export flags registering deleted and groupId so automation can detect successful deletions via --json.

Behavior

• Resolve scope and the target group identifier using the shared helper (numeric ID or case-insensitive name lookup with collision checks).
• Start the progress indicator.
• If --yes is not provided, prompt the user (Delete variable group <name>?). Return util.ErrCancel when declined.
• Call client.DeleteVariableGroup(project, groupID) and treat non-nil errors as fatal.
• Stop progress before printing output. Table output: Variable group deleted. JSON output: { "deleted": true, "groupId": <id> }.
• Add debug logging for resolved IDs and confirmation decisions.

Command Wiring

• Implement internal/cmd/pipelines/variablegroup/delete/delete.go exposing NewCmd(ctx util.CmdContext) *cobra.Command.
• Register it from internal/cmd/pipelines/variablegroup/variablegroup.go so azdo pipelines variable-group delete is available alongside other variable-group commands.
• Regenerate documentation (make docs) after wiring.

SDK / Client Requirements

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

Tooling & Validation

• Format new Go files with gofmt/goimports.
• Run go build ./cmd/azdo/... to ensure the CLI compiles.
• Add/update hermetic tests using mocks (Task Agent client, prompts) and execute go test ./....

Testing

• Unit tests covering:
  1. Successful deletion with --yes.
  2. Prompt decline returning util.ErrCancel.
  3. Lookup by name including collision handling.
  4. SDK error propagation.
  5. JSON export structure.

References

• Azure CLI implementation: variable_group_delete
• Azure DevOps REST API 7.1: Delete variable group