docs: add the ticket save-model v2 spec

The v2 spec folds the structured fields — priority, state, assignee, labels — onto the save engine and removes immediate push, so the package has one write path instead of two. It extends the shipped v1 save-model spec and supersedes the "field setters stay immediate" half of v1 decision 8. I incorporated two Codex review rounds.

I also added the supersession pointer to the top of the v1 spec so the v1/v2 relationship is legible from either file.

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