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>

debian/govpp-snmp-agentx.control.in

@@ -0,0 +1,18 @@
+Package: govpp-snmp-agentx
+Version: @VERSION@
+Architecture: @ARCH@
+Maintainer: Pim van Pelt <pim@ipng.ch>
+Section: net
+Priority: optional
+Depends: snmpd, adduser
+Homepage: https://git.ipng.ch/ipng/govpp-snmp-agentx
+Description: SNMP AgentX subagent that exposes VPP interface statistics
+ govpp-snmp-agentx is a long-running daemon that connects to snmpd via
+ the AgentX protocol and serves the standard IF-MIB (ifTable, ifXTable)
+ from the VPP stats segment. It tracks interface lifecycle events and
+ keeps counters for live interfaces only, using the VPP binary API for
+ metadata and the stats shared memory for fast counter updates.
+ .
+ The daemon runs unprivileged as Debian-snmp:vpp; the systemd unit
+ prepares /var/agentx for the AgentX socket. Configuration of the
+ command-line flags happens in /etc/default/govpp-snmp-agentx.