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

KEYS.md

@@ -23,3 +23,14 @@ Fingerprint:
 Armored export: [`public-key.asc`](public-key.asc)
 
 For policies, ceremonies, and history, see: [`OPENPGP.md`](OPENPGP.md)
+
+
+## `age`
+
+Recipient:
+
+> `age1gzy6tj8y93ljuelcedrkcsv3p6gygk7ng3386u32av9a4ujh737qef8crp`
+
+Recipient file: [`age-recipient.txt`](age-recipient.txt)
+
+For policies, see: [`AGE.md`](AGE.md)