[codex] harden CLI scheduling payloads

[pdurlej]

Summary

• add explicit taskUpdate helpers for Today, Anytime, Someday, Inbox, and date scheduling
• replace auto-Anytime update payloads that used sr: 0 / tir: 0 with explicit null date fields
• keep explicit --scheduled from being cleared when edit also assigns an area, project, or heading
• add CLI payload tests for batch move/edit scheduling semantics

Context

Several update paths used Schedule(1, 0, 0) as shorthand for Anytime while assigning projects, areas, or headings. That is risky because sr=0 and tir=0 can be interpreted as the Unix epoch rather than "no scheduled date". The create path already uses nullable date fields for this distinction; this patch makes sparse update payloads do the same.

Validation

• go test ./cmd/things-cli -count=1
• go test ./...