build(microvm): wire build into deploy + distro + release pipeline

Adds install-microvm Makefile target (with grouped-target staleness
detection on vmlinuz/initramfs.cpio.gz), wires it into dev-install.sh
and reset-dev-env.sh, makes 'make deploy' reinstall artifacts when
present, and makes 'make distro-amd64' stage build/microvm/* into the
release tarball staging dir. arm64 distro stays unchanged (no aarch64
microvm artifact yet — Phase 4).

Release workflow gains a setup-go step and a 'make build-microvm-image'
step before 'make distro-amd64' on the amd64 runner so the resulting
tarball includes the kernel + initramfs without a manual copy.

Precursor for the Phase 3 PC-machine LB removal (mulga-siv-69).
Closes mulga-siv-71.

scripts/dev-install.sh

@@ -95,6 +95,9 @@ echo "=== Building and importing LB image ==="
 cd "$PROJECT_ROOT" && make build-lb-agent
 "$PROJECT_ROOT/scripts/build-system-image.sh" "$PROJECT_ROOT/scripts/images/lb.conf" --import --quiet
 
+echo "=== Building and installing microVM artifacts ==="
+cd "$PROJECT_ROOT" && make install-microvm
+
 echo "=== Done ==="
 echo "Services: sudo systemctl status spinifex.target"
 echo "Logs:     journalctl -u 'spinifex-*' -f"

scripts/reset-dev-env.sh

@@ -273,5 +273,9 @@ echo "==> Building and importing LB image"
 cd "$PROJECT_ROOT" && make build-lb-agent
 "$PROJECT_ROOT/scripts/build-system-image.sh" "$PROJECT_ROOT/scripts/images/lb.conf" --import --quiet
 
+# --- Build + install microVM artifacts (kernel + initramfs for direct-boot LBs) ---
+echo "==> Building and installing microVM artifacts"
+cd "$PROJECT_ROOT" && make install-microvm
+
 # --- Smoke test ---
 sudo -u "$INVOKING_USER" "$SCRIPT_DIR/smoke-test.sh"