Guard Copilot gateway routing without repo context

[ProdByBuddha]

Summary

• derive GitHub Copilot repo/base values from Paperclip workspace realization payloads
• prevent repo-less fallback workspaces from selecting the GitHub Copilot backend
• mark host-lane gateway runs as errors when execution evidence reports failed/timed-out status

Tests

• cargo test github_copilot --quiet
• cargo test gateway_ --quiet
• cargo test --quiet