Rework build system; cut release 1.3.1-1

Replace the dpkg-buildpackage / debhelper rig with the same pattern
used in vpp-maglev: a Makefile that cross-compiles CGO-free static
binaries for amd64 and arm64, plus a debian/build-deb.sh that stages
the .deb directly with dpkg-deb. The two arch packages drop into
build/ and run on any glibc/musl Linux of the matching arch.

VERSION is parsed once from debian/changelog and injected at link
time via -ldflags "-X 'main.Version=...' -X 'main.Commit=...' -X
'main.Date=...'", so 'govpp-snmp-agentx --version' is the source of
truth for which build is running. main.go's Version constant becomes
a var to make this work; the old sync-version make target is gone.

Removes the now-unused debian/{control,rules,postinst,prerm,*.debhelper}
files and adds build/ to .gitignore.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,5 +1,6 @@
 govpp-snmp-agentx
 vppcfg.yaml
+build/
 
 # Debian packaging artifacts
 debian/.debhelper/