Add incremental publishing to a token-free gh-pages Maven repo

[aneveux]

What

Publishes garnish to a token-free static Maven repo on GitHub Pages, modelled on Jenkins Incrementals. Every commit on master becomes a unique, immutable version — no auth needed to consume.

How

• Version scheme — ${revision}${changelist}. Local builds resolve to 0.3.0-SNAPSHOT; CI passes -Dset.changelist and the git-changelist-maven-extension (in .mvn/) rewrites changelist to -rc<commitcount>.<sha>. flatten-maven-plugin bakes the resolved version into the deployed POMs.
• incrementals.yml (new) — on master push: build, test, deploy to gh-pages (fetch-depth: 0, accumulate existing repo, prune, publish via peaceiris).
• prune-maven-repo.py (new) — rolling window of the newest 5 versions, regenerates maven-metadata.xml + checksums.
• release.yml — slimmed to v* tags: GitHub Release with auto notes + update-site link + consumer snippet for the exact version.
• ci.yml — dropped the dead main push trigger (repo uses master); PR gate remains.
• Removed the GitHub Packages distributionManagement; fixed scm url main→master.

Verified locally (Java 25 / Maven 3.9.16)

• Dormant mvn verify → 0.3.0-SNAPSHOT, tests + coverage gate pass.
• -Dset.changelist deploy → 0.3.0-rc11.e435a_05c89b_4, correct Maven2 layout, self-contained flattened POMs + checksums.
• Dirty-tree guard fails as designed.
• Prune script: oldest dropped, 5 kept, metadata + checksums consistent.
• Real consumer resolved the artifact + full transitive Tamboui tree from a plain <repository> with no auth.

Manual step after merge

Settings → Pages → Source = Deploy from a branch → gh-pages / root (after the first incrementals.yml run creates the branch).

Note: 5-version retention means a pinned-but-pruned version stops resolving — a deliberate divergence from real Jenkins incrementals (which keep everything forever). Fine for a personal lib.