mara: 'filling just one of the fields should not touch the other.'
The cpu/mem override form always posts both fields, and the server
correctly treats an empty field as 'clear this override' (documented,
tested behavior in agent_config/resource_limits.rs). The bug was on the
client: the inputs only ever showed the current effective value as a
placeholder, never as the actual value — so a field left untouched
looked filled to the eye but posted empty, silently clearing whatever
override (or lack of one) was already in effect for that field.
Fix: set .value to the current effective value (already fetched and
displayed correctly in the read-only cap columns next to this form) so
leaving a field alone round-trips it unchanged. One resulting nuance
worth flagging: an agent with no per-agent override on a field, that's
purely tracking the hive-wide default, now pins that field to today's
default the first time *any* field on the form is edited, rather than
continuing to silently follow future hive-default changes. Fixing that
fully would need the server to expose whether each field is a real
per-agent override or just the resolved default, which is more than
this bug report asked for.