pm 0.15.9

A pm repo can now review, close and stamp a pull request of another GitHub repo, so one pm project can track work merged in the repos it manages, its own remote on GitHub or not.

Upgrade guide

1. On each machine, install the release: curl -fsSL https://github.com/Yeeef/pm/releases/download/pm-v0.15.9/install.sh | sh.
2. In the repo, run pm upgrade --to 0.15.9, then commit what it changes and merge it, as an ordinary PR.

Added

• pm action need --pr URL takes another GitHub repo's PR: one whose OWNER/REPO the remote's URL does not name, or any GitHub PR when the remote is not on GitHub. The PR must target that repo's default branch, and the design-page check does not run on it, since the pages' covers are this repo's paths.
• pm need close <review> --reason "merged as <sha>", at the close or at a restamp, checks another repo's PR's <sha> against the merge commit gh gives for it into that repo's default branch, instead of this repo's main.
• pm sprint close stamps another repo's PR as OWNER/REPO#N in the Outcome, and --merged SHA --pr URL takes another repo's PR the same way.
• The pm service sees another repo's PR merged into that repo's default branch and tells the session to close its review, without asking it to pull this repo's main.