Migrate build to Mill

[lavrov]

Summary

• Migrated the build from sbt to Mill, following the same approach as the bittorrent repository's migration (commit c4db976): mill wrapper script (pinned via //| mill-version: 1.1.8 frontmatter), build.mill replacing build.sbt/project/, and removal of all sbt files
• protocol is now a JVM-only Mill module. The sbt build cross-published protocolJS, but the bittorrent library no longer publishes Scala.js artifacts (common_sjs1_3 does not exist on Maven Central), so the Scala.js variant could not even resolve its dependencies; the webapp consumes only static assets and does not use protocolJS
• Docker image now runs the Mill assembly jar (./mill server.assembly → out/server/assembly.dest/out.jar) instead of the sbt-native-packager stage directory
• CI: build workflow compiles and assembles via ./mill; release workflow publishes via Mill's SonatypeCentralPublishModule (same as bittorrent, needs MILL_SONATYPE_USERNAME/PASSWORD and MILL_PGP_* secrets)
• Added AGENTS.md with build commands, matching the bittorrent repository

Verification

• ./mill __.compile — SUCCESS (protocol + server)
• ./mill server.assembly — SUCCESS (40MB executable jar)
• Smoke test: java -jar out.jar boots the server, DHT bootstrap runs

Note: the release workflow's secrets must be renamed from SONATYPE_CREDS/PGP_SECRET_KEY to the Mill ones (MILL_SONATYPE_USERNAME, MILL_SONATYPE_PASSWORD, MILL_PGP_SECRET_BASE64, MILL_PGP_PASSPHRASE) before the next tagged release.