test: add copilot network-isolation E2E workflow and register discovery

[Copilot]

This adds a Copilot E2E workflow to cover sandbox.agent.network-isolation: true in workflow_dispatch mode, validating this topology mode is represented in the gh-aw-test suite. It also ensures the test is not silently skipped by e2e.sh default discovery.

• New E2E workflow source

  • Added .github/workflows/test-copilot-network-isolation.md
  • Uses create-issue safe output with samples: for deterministic sample-mode execution
  • Keeps scope minimal: single issue creation path

• Runner discovery wiring

  • Added test-copilot-network-isolation to get_all_tests() in e2e.sh so standard runs include it

sandbox:
  agent:
    network-isolation: true

safe-outputs:
  create-issue:
    title-prefix: "[copilot-network-isolation-test] "
    samples:
      - title: "Hello from Copilot (Network Isolation)"