#11447: feat(providers): add Claude models to GitHub Copilot provider

Add Claude model support (claude-sonnet-4, claude-opus-4.5) to the GitHub Copilot provider, enabling users to access Anthropic models through their Copilot subscription. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR expands the built-in `github-copilot` provider by adding Claude model IDs to the default Copilot model list and documenting the available Copilot models. It also tweaks failover classification to treat the string `fetch failed` as a timeout. The main issues are around model-id correctness/consistency: the PR introduces `claude-sonnet-4.5` (dot) but the rest of the repository uses the dash form `claude-sonnet-4-5`; if Copilot expects the dash form, the new default will fail and the docs will cause users to copy an invalid ID. Additionally, treating generic `fetch failed` network errors as timeouts can trigger failover for non-timeout failures, altering behavior in cases like DNS/TLS/connection failures. <h3>Confidence Score: 3/5</h3> - This PR is close to mergeable but likely needs fixes to model IDs and error classification behavior. - Core changes are small, but the introduced `claude-sonnet-4.5` identifier appears inconsistent with established model-id conventions in this repo and may cause immediate runtime model-selection failures if Copilot expects `claude-sonnet-4-5`. Separately, classifying the generic undici `fetch failed` error as a timeout can change failover routing for non-timeout network failures. - docs/providers/github-copilot.md, src/providers/github-copilot-models.ts, src/agents/pi-embedded-helpers/errors.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->