feat: add spinloop work logs #231

Read an item's kept agent output through the work list API's existing GET /v1/items/{id}/log path — the same client-of-the-API shape as add, list, abort and remove (#228). There is currently no way to read an item's log except by reaching the orchestrator's filesystem directly, which defeats the point for a remote client (a GitHub action, a CI job) that only has network access.

• spinloop work logs <id> --url <address>, the same --api-token/--api-token-file/SPINLOOP_API_TOKEN resolution the rest of the family uses
• An item with no output yet is answered empty, not a fault, matching the API's own log: null answer
• -f/--follow polls for new output and streams it as it arrives, the way tail -f does, ending when the item's state is no longer running (or on the operator's interrupt)
• An id the run does not carry is refused, naming it, the way the other subcommands do

Follows on from #228.