docs: Loops 2.0 rewrite spec (#611)

* docs: specify Loops 2.0 rewrite

* docs(loops): drop v1 compatibility — replace, don't migrate

v1 Goal Runs has no active users or preserved history, so the spec no
longer carries a normalization layer, legacy renderer, or dual-API
window. Rephased delivery accordingly (durable engine core first) and
fixed the restart-behavior description (runs are marked aborted, not
failed).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: alari76