provider/sourcehut: add builds.sr.ht provider
Submits jobs via GraphQL `submit`, polls for status, streams per-task
logs from /query/log/ (the path that bypasses sourcehut's anti-bot
proxy). Workflows opt in via `tack.sourcehut.manifest`; tack injects
TACK_* env vars into the manifest before submission.
Signed-off-by: Jes Olson <j3s@c3f.net>