cache: propagate github instance slug

[max-blacksmith]

Cache requests need the GitHub instance identity so matching owner/repo names on different instances cannot share entries.

Read BLACKSMITH_INSTANCE_SLUG from the runner environment and send X-Blacksmith-Instance-Slug on restore, reserve, and commit requests. Missing values default to github for existing dotcom runners.

flowchart LR
  job[Job instance_slug] --> agent[Agent guest environment]
  agent --> toolkit[Cache HTTP client]
  toolkit --> header[X-Blacksmith-Instance-Slug]
  header --> backend[Instance-scoped backend]

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Targets useFetchAsHTTPClient, the branch with the cache package used by the actions. Two header propagation tests passed on Testbox; Prettier passed.

Deploy the backend and agents first. After merging, publish the cache package, update pinned dependencies in the cache/setup actions, and rebuild their bundles before enabling non-dotcom caching.

Companion PRs: backend, agent.

[max-blacksmith]

This information shouldn't be passed like this.