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

ceremonies/2026-09-13_age_key.md.asc

@@ -0,0 +1,7 @@
+-----BEGIN PGP SIGNATURE-----
+
+iHUEABYKAB0WIQTW0Zid8fayP9d2KT20bttN/CaAXQUCaqcOrAAKCRC0bttN/CaA
+Xaj9AQD+Wwuxhlserdi+HGO5XJabm4xCEcVqCQid+zQctG97hQEAxBBSzzL6yNwG
+4pHqSmaMUvJkIkSSBH4ERxSsGvvjGw4=
+=h7yN
+-----END PGP SIGNATURE-----