Make in-place upgrade from HashiCorp Boundary seamless

Linux packages: migrate /etc/boundary.d to /etc/threshold.d without
clobbering, reuse the boundary service user, carry over service
enablement with a boundary.service alias, provide a boundary command.
Docker: accept /boundary/config.hcl, boundary command, /boundary symlink.
CLI: deprecation notice when invoked as boundary (stderr only).
Dev mode: also create *.threshold.dev aliases.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -282,6 +282,7 @@ jobs:
           rpm_depends: "openssl"
           config_dir: ".release/linux/package/"
           preinstall: ".release/linux/preinst"
+          postinstall: ".release/linux/postinst"
           postremove: ".release/linux/postrm"
       - name: Add Linux Package names to env
         run: |