Docs: Fix duo-cli package name and add offline flow execution guide

What does this MR do?

Two changes to doc/user/duo_agent_platform/flows/execution.md:

1. Typo fix: Line 146 referenced @gitlab-org/duo-cli which does not exist on npm (404). Corrected to @gitlab/duo-cli, consistent with every other reference in the file and the Rails source (start_workflow_service.rb).

2. New section: "Use a custom image in an offline environment" documents how to run DAP Flows in air-gapped environments where runners cannot reach npmjs.org. Two verified paths:

   • Pre-bake @gitlab/duo-cli via npm at image build time
   • Use the standalone binary from the GitLab Generic Package Registry (no npm required at all)

   Also covers image transfer (docker save/docker load), the admin custom image registry setting (shipped 18.9, #584689 (closed)), and agent-config.yml configuration.

Both paths were tested on GCE (Fedora 42, podman, --network=none) on 2026-04-11 and confirmed working. Test results posted on #582126.

Related issues

• #582126 (does not close -- issue tracks a product-level solution; this MR documents the workaround)
• !219672 (merged) (the conditional command -v duo check that makes the pre-bake workaround possible, shipped 18.9)

Author's checklist

• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Not adding a new page, so availability details are not needed.
• Request review based on page metadata (AI-powered / Agent Foundations).

AI-Generated Content Disclosure: This MR contains documentation drafted with assistance from Claude Code. The content was verified by lab testing on GCE with network disabled. Test methodology and results are documented on #582126.