Feature request : First-class atom expiry: valid_until field, expired-atoms view, optional surfacing notification

[Sainin701]

Problem :

Knowledge doesn't age uniformly. Some atoms are eternal (biology basics,
durable frameworks). Others decay fast (platform-specific tactics,
time-bound market conditions, current events). There's no way to tell
Atomic "this atom is only valid until X date" and have the app do
anything with that information.

Example use case :

In fast-moving domains (performance marketing platform tactics, crypto
market conditions, current health research), I capture content that's
valid for a quarter or less. Today I tag these atoms with
valid-until/2026-06-22 as a convention, but:

• The app doesn't surface atoms whose expiry has passed
• I can't filter "show me all expired atoms"
• Nothing reminds me to review and refresh or archive

So my chat confidently cites a 2023 platform tactic as current
advice in 2026.

Proposal

Add an optional valid_until field on atoms (date, or auto-parsed from
a valid-until/YYYY-MM-DD tag for backward compat). Then:

1. An "Expiring soon / expired" view in the sidebar (atoms past
   valid_until, or within N days of it)
2. A filter on search/chat scope: valid_until:not-expired
3. Optional: include expired atoms in the daily briefing as a
   "needs review" section