prime servings jq examples reference 'Day' field; actual JSON field is 'RecordedTime'

[DTTerastar]

`crono-export prime` shows jq recipes against a `Day` field on servings records, but the actual JSON output uses `RecordedTime`. `Day` is null on every row.

Caught during pre-release sanity check on commit `3792cdc`.

Severity: minor (cosmetic — wrong recipe in docs)

[DTTerastar]

@claude Please fix

[claude]

Claude finished @DTTerastar's task in 10m 12s —— View job • claude/issue-25-20260426-0039 • Create PR ➔

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Fixing jq examples: Day → RecordedTime

• Read CLAUDE.md for repo-specific instructions
• Locate jq example strings referencing Day field
• Add date-based servings jq example using .RecordedTime
• Add GOTCHA about Day being null in servings JSON
• Commit and push the fix

Changes made to cmd/prime.go:

1. New jq recipe — added a concrete date-filtering example for servings using .RecordedTime:

   # All servings on a specific day
   crono-export servings --since 7d --format json | jq '[.[] | select(.RecordedTime | startswith("2026-04-25"))]'
   

2. New GOTCHA — warns that the Day field in servings JSON is always null and directs users to RecordedTime:

   - 'servings' JSON rows contain a 'Day' field that is always null — use
     'RecordedTime' (an ISO-8601 timestamp) for any date-based filtering.