bird: add learn + explicit static blackhole protocols

BIRD2's protocol kernel does not import kernel routes by default; the
import filter on the channel is just for what BIRD has already learned.
Added `learn;` so the kernel-installed blackholes (from the agent's
SummaryRoutes) are picked up.

Also added explicit `protocol static static6/static4` with one
`route <cidr> blackhole;` per NodeConfig CIDR. This is belt-and-
suspenders: even if `learn` doesn't capture the kernel blackhole, BIRD
has the route directly and exports it via the BGP filter.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

pkg/routing/bird/config.go

@@ -43,17 +43,30 @@ protocol device { scan time 10; }
 protocol direct { interface "lo"; }
 
 protocol kernel kernel6 {
+  learn;
   ipv6 {
     import all;
     export all;
   };
 }
 protocol kernel kernel4 {
+  learn;
   ipv4 {
     import all;
     export all;
   };
 }
+
+protocol static static6 {
+  ipv6;
+  {{range $cidr := .CIDR6}}route {{$cidr}} blackhole;
+  {{end}}
+}
+protocol static static4 {
+  ipv4;
+  {{range $cidr := .CIDR4}}route {{$cidr}} blackhole;
+  {{end}}
+}
 {{range $i, $p := .Peers}}{{if eq $p.Family "v6"}}
 protocol bgp upstream6_{{$i}} {
   local as {{$.LocalASN}};