build: add axion-release versioning, Spotless formatter, GHCR publish workflow

[ex172000]

Summary

Closes the three open-source readiness gaps before the public switchover:

• Versioning — adds pl.allegro.tech.build.axion-release (1.18.16). Version is derived from git tags; the hard-coded 1.0.0-SNAPSHOT is removed. ./gradlew currentVersion reports 0.1.0-SNAPSHOT today; the first official release pins 1.0.0 via ./gradlew release -Prelease.version=1.0.0.
• Publishing — new .github/workflows/release.yml triggered on v*.*.* tags. Builds + pushes a multi-arch (linux/amd64, linux/arm64) Docker image to ghcr.io/uber/ugroup with tags :X.Y.Z, :X.Y, :X, :latest, and creates a GitHub Release with the boot jar attached.
• Formatter — adds com.diffplug.spotless (6.25.0) with googleJavaFormat('1.22.0'), removeUnusedImports, trailing-whitespace + newline normalization, and an Apache 2.0 license header (gradle/license-header.txt). spotlessCheck runs as part of ./gradlew build.
• README rewrite — lean, with Quick Start, REST examples, the verified ugroup_consumer_lag PromQL, and the three watchlist modes. Deep dives stay on the Wiki.
• CONTRIBUTING — adds the ./gradlew spotlessApply step and .git-blame-ignore-revs setup note.

Merge order

This PR adds spotlessCheck to the default build but the existing Java code is not yet formatted. Merge #style/spotless-reformat first to avoid a temporarily red main, then merge this. After both land, fill the reformat commit SHA into .git-blame-ignore-revs in a follow-up commit.

Test plan

• ./gradlew currentVersion → 0.1.0-SNAPSHOT
• ./gradlew build -x integrationTest passes (Spotless + Spotbugs + JaCoCo + unit tests)
• Tag-triggered release workflow dry-run with a throwaway v0.0.1-rc1 tag pushes to GHCR (verify before the real v1.0.0)

🤖 Generated with Claude Code

[yfeng21]

exact year missing

[ex172000]

$YEAR is a placeholder Spotless's licenseHeaderFile recognizes. It tells Spotless where the year sits in the template so it can match existing headers and decide how to fill it in based on yearMode.

[yfeng21]

Why do we need this file, nothing is included

[ex172000]

It exists so git blame doesn't attribute every line in the codebase to the Spotless reformat commit.