Stamp build identity into do-obsd via ldflags

[jnaidu-do]

Summary

• Bake the binary's version into do-obsd via -ldflags "-X main.version=$(VERSION)". revision and buildDate come from runtime/debug.ReadBuildInfo() (auto VCS embed; no extra ldflags).
• Add a -version CLI flag and enrich the startup slog line so a running supervisor self-identifies (version, revision, buildDate, goVersion).
• Symlink /usr/bin/do-obsd → /opt/digitalocean/bin/do-obsd in the post-install scriptlet so operators can run do-obsd -version from PATH; cleaned up on uninstall.
• Makefile gets a VERSION ?= override hook for the cthulhu release pipeline, plus -s -w and -trimpath to align with what CI was already doing.

Test

• Verified by deploying the build on a droplet and executing --version command