Servings JSON 'Day' field is always null — dead schema field #43

QA finding: every row of crono-export servings --format json includes a Day field whose value is always null:

$ crono-export servings --since 90d --format json | jq '[.[] | .Day] | unique'
[
  null
]

This is documented in the prime GOTCHAS ("'servings' rows have a 'Day' field that is always null — use 'RecordedTime'"), so it's a known wart — but documenting it doesn't make it less wrong. Consumers waste a key on a sentinel; jq recipes that introspect schemas have to special-case it.

Two reasonable fixes:

1. Drop the field from ServingRecord (json tag with omitempty or remove entirely). Cleanest.
2. Populate it from RecordedTime's local-date portion if it's meant to be a denormalized convenience. Keeps the field but makes it useful.

I lean #1 — RecordedTime already carries the info, the GOTCHAS note already tells users to use it.

Once fixed, remove the corresponding line from cmd/prime.go GOTCHAS.

Severity: trivial (cosmetic schema cleanup)