Add age key

- Add AGE.md with policies
- Add age-recipient.txt for `age -R`
- Add ceremonies/2026-09-13_age_key.md as the signed record
  of which recipient is valid from which date
- List the recipient in KEYS.md
- Describe AGE.md and age-recipient.txt in README.md
- Extend the scripts:
  + sign.sh signs AGE.md by default
  + verify.sh verifies AGE.md by default
    and cross-checks age-recipient.txt,
    the only unsigned artifact,
    against the recipient in the signed KEYS.md and AGE.md

README.md

@@ -38,6 +38,7 @@ gpg --import public-key.asc
 
 gpg --verify KEYS.md.asc KEYS.md
 gpg --verify OPENPGP.md.asc OPENPGP.md
+gpg --verify AGE.md.asc AGE.md
 ```
 
 Each `gpg --verify` will print two things:
@@ -96,6 +97,22 @@ How I implement [OpenPGP](https://www.openpgp.org/):
   Armored public keys of all previous OpenPGP keys
 
 
+### `age`
+
+Besides OpenPGP,
+    I also use the modern [`age`](https://github.com/FiloSottile/age) encryption:
+
+- [`AGE.md`](AGE.md):
+  `age` policies
+- [`AGE.md.asc`](AGE.md.asc):
+  Detached signature, renewed at every ceremony
+- [`age-recipient.txt`](age-recipient.txt):
+  `age` public key for `age -R`
+- [`ceremonies/`](ceremonies/):
+  Signed record of when a recipient became valid or rotated
+  because `age` keys carry no metadata of their own
+
+
 ### Scripts
 
 `sh` scripts for my own use:
@@ -107,7 +124,8 @@ How I implement [OpenPGP](https://www.openpgp.org/):
   Renews the detached signatures
 - [`verify.sh`](scripts/verify.sh):
   Verifies all signed files against the pinned signing subkey,
-  and warns when the key is about to expire
+  warns when the key is about to expire, and
+  cross-checks `age-recipient.txt`
 - [`snapshot.sh`](scripts/snapshot.sh):
   Exports the current key to `snapshots/` and `public-key.asc`
 - [`publish.sh`](scripts/publish.sh):