Introduce azdo pipelines agent command group

[tmeckel]

Introduce the azdo pipelines agent subgroup under the pipelines umbrella (#116). This subgroup manages Azure DevOps build and deployment agents.

Org-scoped, not project-scoped. Agents live inside agent pools, which are org-scoped. The agent subgroup therefore takes a positional ORGANIZATION (or falls back to the default), identical to the pool subgroup (#239) and the pool show sibling (#244).

CLI Structure

azdo pipelines agent
├── list  (sub of this issue; spec #248)
└── show  (sub of this issue; spec #247)

Scope

Mirrors az pipelines agent. All commands reuse the vendored taskagent.Client:

• taskagent.Client.GetAgents — list agents in a pool (spec feat: Implement azdo pipelines agent list command #248)
• taskagent.Client.GetAgent — show a single agent by pool+id (spec feat: Implement azdo pipelines agent show command #247)

Both mocks are already generated at internal/mocks/taskagent_client_mock.go:336-345 (GetAgent) and :531-540 (GetAgents). No mock regeneration needed.

Implementation Outline

1. Create internal/cmd/pipelines/agent/agent.go (umbrella) with Use: "agent", Aliases: []string{"agents"}.
2. Register it from internal/cmd/pipelines/pipelines.go after variablegroup (preserving order of registration so help text remains stable).
3. First leaves: list (feat: Implement azdo pipelines agent list command #248) and show (feat: Implement azdo pipelines agent show command #247) — each implemented in their respective sub-issues.

References

• az pipelines agent (Azure CLI reference for UX)
• internal/mocks/taskagent_client_mock.go — mocks for GetAgent and GetAgents (already generated).
• internal/azdo/factory.go:133-139 — ClientFactory().TaskAgent(...) accessor.
• Sibling subgroups: pipelines/pool (Introduce azdo pipelines pool command group #239) and pipelines/queue (Introduce azdo pipelines queue command group #240) use the same client.

Sub-issues

• feat: Implement azdo pipelines agent list command #248 — feat: Implement azdo pipelines agent list
• feat: Implement azdo pipelines agent show command #247 — feat: Implement azdo pipelines agent show