Add Copilot assign-to-agent E2E workflow coverage

[Copilot]

This adds missing E2E coverage for the Copilot assign-to-agent safe output introduced by recent gh-aw changes. The new test exercises assignment via a temporary issue reference and uses ignore-if-error: true to keep the workflow non-flaky when agent availability is restricted.

• New workflow test (workflow_dispatch)

  • Added .github/workflows/test-copilot-assign-to-agent.md.
  • Uses create-issue with temporary_id: aw_agent_issue, then assign-to-agent with issue_number: "#aw_agent_issue".
  • Configures allowed: [copilot] and ignore-if-error: true.

• Compiled workflow artifact

  • Added generated lockfile .github/workflows/test-copilot-assign-to-agent.lock.yml.

• Runner and coverage wiring

  • Added test-copilot-assign-to-agent to get_all_tests() in e2e.sh so it participates in normal suite execution.
  • Updated README safe-output coverage: marked assign-to-agent as covered and removed it from deferred infrastructure-only items.

safe-outputs:
  create-issue:
    samples:
      - temporary_id: aw_agent_issue
        title: "Test assign to agent from Copilot"
  assign-to-agent:
    target: "*"
    allowed: [copilot]
    ignore-if-error: true
    samples:
      - issue_number: "#aw_agent_issue"