Superseded memories are kept forever: let them expire or be cleaned up #441

Since #436 (#152), a memory replaced by a newer one (for example "lives in Pune" after "moved to Bengaluru") is kept as superseded history instead of being deleted. It's left out of prompts and only listed when asked for. Nothing ever removes it, though, so they pile up, and a user who wants something gone has to find the old version too.

Idea:

• a config option such as memory.keep_superseded_days (default 180, 0 = forever), with a description so Settings shows it;
• the nightly memory job deletes superseded memories older than that, through the same cascading delete feat(memory): source links, confidence and state, and deletes that cascade #436 added (vectors, links, summaries, insights' evidence);
• deleting a current memory offers to delete its earlier versions too.

Where: sentient/memory/facts.py (states and cascading delete from #436), the nightly job in sentient/memory/dreaming.py, and config/schema.py.

Done when: a superseded memory older than the setting is gone after the nightly run, along with everything derived from it, and current memories are untouched. Add tests for both.