feat(backlog): flesh the defer mechanics, pre-flight Q&A, and page

The defer checklist gained its filing mechanics. A deferral VERIFY now dedups against an existing sibling before filing, since the deferred task stays TODO and every later run would otherwise re-file. Placement, heading, and body follow the todo conventions.

A quick-question gap routes to the pre-flight Q&A only under the speedrun preset, and only for one-line factual or preference picks. Three or more questions means the task is underspecified and files instead. The data-loss item never routes to the Q&A.

The batch-ask is one message with recommendation-first numbered options, and answers land as dated lines in the task bodies before the run starts. The page fires exactly once, on set-done or cap-hit, via notify --persist.

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