Add generated MockTaskAgentClient and wire TaskAgent into the mock
client factory. Update the mock generation script to produce the new
taskagent mock file so tests can stub TaskAgent API calls.

Add a raw REST client for the Azure DevOps variable groups endpoint to
work around the SDK's mis-shaped response for variable groups.This enables listing variable groups (with continuation) reliably when the taskagent SDK wrapper is insufficient.

Introduce support for the Azure DevOps taskagent v7 client by:
- adding the taskagent import and TaskAgent method to the ClientFactory
- implementing TaskAgent in the client factory to create a taskagent client from a connection

Add a GetVariableGroups helper to internal/azdo/loader.go that uses the
taskagent client to list all variable groups for a given project.

Add a new top-level `pipelines` command and a `variable-groups list`
subcommand to list Azure DevOps variable groups for a project.

Add new documentation pages for the `azdo pipelines` command and the
`azdo pipelines variable-groups` subcommands, including a detailed
`list` command reference.

Clarify how commands should register and produce JSON output. Explicitly
require the slice passed to util.AddJSONFlags to enumerate every JSON field
(matching struct tag names) so consumers can filter reliably.