Seeds AlertConfig.DiskFillByType in defaultAlertConfig() with the three
forward-compatible keys (nvme 92/87, sata 90/85, hdd 85/80). Adds
NormalizeDiskFillByType to lowercase keys on load, seed defaults when
the map is nil, and reset non-positive trigger/clear values to the
default for that key. NormalizeAgentDefaults invokes the new helper so
it runs through the existing UpdateConfig normalization chain. Round
trip tests cover nil seed, customized survival, negative reset, and
mixed case normalization, plus a defaultAlertConfig seed assertion.
Only the "nvme" key is consulted by the host evaluation today (the
inference helper only matches /dev/nvme*); "sata" and "hdd" are
forward-compatible placeholders for when the agent protocol carries
hardware type explicitly.
rcourtman