Reusing session_id as document_id caused data loss on /resume: when
the session is loaded again, _session_turns starts empty and the next
retain replaces the entire previously stored content.
Now each process lifecycle gets its own document_id formed as
{session_id}-{startup_timestamp}, so:
- Same session, same process: turns accumulate into one document (existing behavior)
- Resume (new process, same session): writes a new document, old one preserved
- Forks: child process gets its own document; parent's doc is untouched
Also adds session lineage tags so all processes for the same session
(or its parent) can still be filtered together via recall:
- session:<session_id> on every retain
- parent:<parent_session_id> when initialized with parent_session_id
Closes#6602
f9c6c5ab84