fix(deepagents): exclude summarization state from subagent input/output (#749)

Fixes #748 

Excludes `_summarizationEvent`/`_summarizationSessionId` from subagent
state input/output. These weren't in `EXCLUDED_STATE_KEYS`, so a stale
parent cutoffIndex could silently drop a subagent's task description,
and a subagent's own summarization event could overwrite the parent's on
return.