Add security.txt

- Add scripts/security-txt.sh
  to generate a security.txt file
- Add the current security.txt
- Extend verify.sh to verify security.txt by default
  and cross-check that it expires together with the signing key
- Describe security.txt in OPENPGP.md and README.md

OPENPGP.md

@@ -65,6 +65,8 @@ gpg --verify OPENPGP.md.asc OPENPGP.md
   - `public-key.asc`, the armored export of the current PGP key,
     for convenient usage,
     refreshed by `scripts/snapshot.sh`
+  - `security.txt`, clearsigned, expiry matches the key,
+    refreshed by `scripts/security-txt.sh`
   - `ceremonies/`, signed records of exceptional events,
     such as the 2026 supersession and the `age` key introduction
   - `history/`, armored exports of all retired keys,
@@ -75,7 +77,7 @@ gpg --verify OPENPGP.md.asc OPENPGP.md
     written by `scripts/snapshot.sh`
   - One commit per ceremony to keep
     snapshot, updated tables,
-    and re-signed `KEYS.md`, `OPENPGP.md`, and `AGE.md` together
+    and re-signed `KEYS.md`, `OPENPGP.md`, `AGE.md`, and `security.txt` together
 - Distribution:
   - WKD on all three UID domains
   - Public keyservers (see section below)
@@ -181,11 +183,12 @@ gpg --list-secret-keys <FPR>
 Then, in this repo:
 
 1. `./scripts/snapshot.sh`
-2. Update the dates in the two tables above
-3. `./scripts/sign.sh`
-4. `./scripts/verify.sh`
-5. `./scripts/publish.sh`
-6. One commit
+2. `./scripts/security-txt.sh`
+3. Update the dates in the two tables above
+4. `./scripts/sign.sh`
+5. `./scripts/verify.sh`
+6. `./scripts/publish.sh`
+7. One commit
 
 Then, on every other machine: