Amp Runners Now Serve Multiple Directories and Update Themselves

Amp runners can now serve multiple directories, specified with repeated --dir flags or found automatically with --discover-dirs, which scans Git checkouts up to two levels deep by default.

Runners also check for new releases about once an hour, install them, and restart into the new version once no thread is running, at most once every 12 hours, with auto-update disabled via amp.runner.autoUpdate.enabled: false.