Two new context fields for upstream hooks (e.g. security plugins) to
control session memory persistence:
- blockSessionSave (boolean): prevent session from being saved to memory
- sessionSaveContent (string): override saved content with custom text
(empty string is valid — persists a blank marker without transcript)
When sessionSaveContent is set, LLM slug generation and session content
loading are skipped (unnecessary when content is overridden).
Split from #35567 — sessionSaveRedirectPath follows separately as it
requires path canonicalization, symlink resolution, and filesystem
write policy review.
65fae19fc8