fix(review): stop contacting the git remote from the idle freshness poll; add PLANNOTATOR_GIT_REMOTE_CHECK opt-out and kill orphaned git transports on exit (#1585)

During an idle code review the server ran git ls-remote against origin once a minute for as long as the page was open, and orphaned the git/ssh child on shutdown; on hosts whose SSH auth goes through a hardware key that meant a touch prompt every minute. The remote is now contacted at startup and on a deliberate diff switch, refresh, or fetch, never from the 5-second freshness poll; a failing probe backs off up to 15 minutes; PLANNOTATOR_GIT_REMOTE_CHECK=0, { gitRemoteCheck: false } in config.json, or plannotator review --no-git-remote-check disables all remote contact; and isolated git children are killed on exit. Both runtimes. A push to the base branch that lands mid-review now surfaces on the next refresh instead of within the minute. Closes #1553.