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

@@ -0,0 +1,25 @@
+# New `age` Key
+
+Effective as of 2026-09-13,
+    I publish an `age` identity next to my OpenPGP key:
+
+```
+age1gzy6tj8y93ljuelcedrkcsv3p6gygk7ng3386u32av9a4ujh737qef8crp
+```
+
+Type X25519, created 2026-09-11, no expiry and no revocation.
+
+No OpenPGP key material changed in this ceremony.
+`AGE.md` was added, `KEYS.md` gained the recipient,
+    and all detached signatures were renewed.
+
+Because `age` recipients carry no metadata,
+    this signed record is the only proof
+    of which recipient was valid from which date.
+
+A future rotation gets its own record here,
+    superseding this one.
+
+The policies are in [`AGE.md`](../AGE.md).
+
+This file is signed with the current OpenPGP key.

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-----