fix: CI — remove unused import, fix flaky test assertion, bump version to 3.10.0

- Remove unused `ChannelMessage` import in github-copilot-plugin.ts (lint error)
- Fix `anthropic.test.ts`: system prompt says "Just the poem" so asserting
  `output.contains("shakespeare")` is wrong — the model correctly returns only
  the poem content without mentioning the author
- Apply all pending changesets via `changeset version` to bump braintrust to
  3.10.0, so the API compatibility test detects a `minor` version bump rather
  than `none` and does not flag optional interface additions as breaking

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Qard