Name snapshots by full date instead of just year

Snapshots are now written as <KEYID>-public_YYYY-MM-DD.asc,
one file per ceremony, matching the naming of the secret exports
kept outside this repository

OPENPGP.md

@@ -71,9 +71,8 @@ gpg --verify OPENPGP.md.asc OPENPGP.md
     such as the 2026 supersession and the `age` key introduction
   - `history/`, armored exports of all retired keys,
     see [Key History](#key-history) below
-  - `snapshots/<KEYID>-public-YYYY.asc`,
-    refreshed at every ceremony,
-    one file per year,
+  - `snapshots/<KEYID>-public_YYYY-MM-DD.asc`,
+    one file per ceremony,
     written by `scripts/snapshot.sh`
   - One commit per ceremony to keep
     snapshot, updated tables,