State what the default blocklist covers (closes #244)

The default blocklist covers the IPv4 private and reserved ranges, IPv6
loopback, unique local and link-local addresses, and public addresses
that hand credentials to whatever can reach them, without the caller
presenting any. A provider's other services on public addresses, such
as 161.26.0.0/16 and 166.8.0.0/14, are deliberately not on it: they
hand out no credentials that way, reaching them can be legitimate, and
every cloud has some, so a partial list would promise coverage it does
not give.

The README's egress section and the comment above blockedNetworks now
state this rule, so nobody infers wider coverage and a future candidate
can be accepted or refused against it. No list change.

Model: opus-5-5

README.md

@@ -162,6 +162,20 @@ public cloud metadata addresses: currently only `168.63.129.16`, Azure's
 WireServer, which serves an Azure VM its credentials. Because it is a
 public address, listing it in `ALLOWED_EGRESS_CIDRS` reopens it.
 
+That is all the default blocklist covers: the IPv4 private and reserved
+ranges; of IPv6, only loopback (`::1`), unique local addresses
+(`fc00::/7`) and link-local addresses (`fe80::/10`); and public
+addresses that hand credentials to whatever can reach them, without the
+caller presenting any. A cloud provider's other services on public
+addresses are not refused. IBM Cloud, for example, serves its DNS
+resolvers, package mirrors, time servers and object storage on
+`161.26.0.0/16`, and the private endpoints of its own cloud services on
+`166.8.0.0/14`. Neither range hands out credentials that way: the token
+service among those endpoints issues a token only in exchange for
+something the caller presents, such as an API key. Reaching these
+services can be a legitimate delivery, and every cloud has some, so a
+partial list would promise coverage it does not give.
+
 That default is also inconvenient for the thing webhooker is mostly
 for: taking a public webhook and forwarding it to something on your own
 network. A container on the same Docker network, a box on `10.x`, a

internal/delivery/ssrf.go

@@ -43,6 +43,12 @@ var (
 // permit specific blocks out of this set with
 // ALLOWED_EGRESS_CIDRS; see Guard.
 //
+// A public address belongs here only if it hands credentials to
+// whatever can reach it, without the caller presenting any; a
+// provider's other services on public addresses, such as its DNS
+// resolvers or package mirrors, stay out, since reaching them can
+// be legitimate and no list of them could be complete.
+//
 //nolint:gochecknoglobals // package-level network list is appropriate here
 var blockedNetworks []*net.IPNet