Task due dates live in the checklist line

Context

WorkspaceTask has carried a dueDate since ADR-0031, but nothing set it. A date that only exists in SQLite would disappear from the note's Markdown, so an exported vault, a Git projection, or another editor would lose it. ADR-0031 already makes the source document own a linked task's title and completion; a second field with a different owner would give the record and the checklist item two ways to disagree.

Decision

• The date is a dueDate attribute on check_item, a YYYY-MM-DD day with no time or zone. Unlinked checklist items may carry one too; like any checkbox, that stays document-only content and never creates a task.
• Markdown spells it as the Obsidian Tasks due-date signifier at the end of the item's first line, after the private task marker: - [ ] Ship it <!--skriuw-task:t:b--> 📅 2026-10-01. Obsidian Tasks only reads signifiers at the end of the line, so the date goes last. The parser reads marker and date in either order and leaves a token that names no real day as literal text.
• The document owns the date. Every save reconciles a linked task's due_date from its checklist item, including clearing it, exactly as it reconciles the title and checkbox. Setting a date from the tasks view is a paired update_task that rewrites the item in the same operation.
• The renderer mirrors that reconciliation when it projects a save, so the tasks view follows edits made in the editor without waiting for a snapshot.
• In the editor, typing due:<date> and a space on a checklist line sets the date; the chip it renders turns back into that token on Enter or click and clears on Delete.

Consequences

• No migration: workspace_tasks.due_date already exists and archives already export it.
• A due date stored only on the record, for example from an archive written before this decision, is cleared by the next save of its source note.
• Other tools' date conventions (due: in todo.txt, DEADLINE: in Logseq) are not read from Markdown. Typing due: is an editor gesture, not a file format.