* feat(flow): add optional key param to @persist decorator
Allows users to specify which state attribute to use as the
persistence key instead of always defaulting to state.id.
Usage: @persist(key='conversation_id')
Falls back to state.id when key is not provided (no breaking change).
Raises ValueError if the specified key is missing or falsy on state.
* docs(flow): document @persist key parameter for custom persistence keys
* fix(flow): use explicit None check for persist key to avoid empty-string fallback
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Co-authored-by: iris-clawd <iris-clawd@anthropic.com>
Co-authored-by: iris-clawd <iris@crewai.com>
Co-authored-by: Lorenze Jay <63378463+lorenzejay@users.noreply.github.com>
