Run gh subprocesses async so a slow fetch cannot freeze the server

[fiddur]

Every git/gh call is execSync/execFileSync; a slow gh api fetch (e.g. /api/github/details) blocks the event loop — viewer heartbeats, the live-claim watcher, and every poll stall with it.

Proposal: async execFile for the gh boundary first (it is network-bound); git can stay sync until it hurts.

Effort: M.