refactor(build): compile the examples as their own module - #475

The examples were a second source root of the library, added by build-helper in the default profile. Once module-info.java landed in src/main/java, javac compiled them as part of module se.michaelthelin.spotify, so their root packages (data.*, authorization.*) ended up inside the module. The release profile has no build-helper, so the published module never contained them: mvn package produced a 3351-byte descriptor against 2828 from -P release. CI was validating a module that was never published, and the CI jar shipped 115 example classes that were not meant to be in it.

They are now their own Maven project with their own module descriptor, compiling against the installed library rather than alongside it. Both profiles produce a 2828-byte descriptor and the jar carries no example classes.

Making them a separate module buys something the previous arrangement could not. examples/module-info.java requires only se.michaelthelin.spotify, so everything else they touch, Gson and HttpComponents among it, has to reach them through the library's own requires transitive. A package the library forgets to export, or a requires that should have been transitive, now fails this build instead of someone else's. Deleting a single exports line from the library and rebuilding gives package se.michaelthelin.spotify.enums is not visible across six examples, which is a class of mistake nothing caught before.

The files stay exactly where they are. README.md links all 115 of them by path and ci/check-docs.py validates those links, so the project points sourceDirectory at examples/ and sends build output to target/examples rather than moving everything under a conventional src/main/java and rewriting the index.

CI installs the library and then runs mvn -f examples/pom.xml clean compile. The clean is load-bearing: an incremental build reuses classes compiled against the previous descriptor and reports nothing, which is how the first version of this passed when it should have failed.

The examples version is a third place the release version lives, so ci/check-docs.py now fails when it falls behind pom.xml, and AGENTS.md records both that and how to build the examples.

Build-only, no library code changes: 402 tests, 116 example sources compiled on the module path, check-docs clean and javadoc without warnings.

CONTRIBUTING.md is brought back in line at the same time, because this change is one of the things that dated it. Its release section named the wrong number of files to bump and said nothing about ci/check-docs.py now catching a missed one; its documentation rule asked for complete Javadoc, which contradicts a codebase that deliberately leaves model builder setters undocumented and runs doclint as all,-missing; it pointed at Spotify's live documentation for questions spec/openapi.yaml now answers; and it described neither the module descriptor, though a class in a new package is unreachable until that package is exported, nor which branch a contribution belongs on. That last one is now spelled out: main takes anything that can ship in a patch or minor release, new endpoints and parameters included, while beta takes changes that break the published API and accumulates the next major; pre-releases are tagged from beta and stable releases from main.

AGENTS.md is the file agents load at startup and it never referenced the guide at all, so it now links to it up front and defers the release steps to it rather than keeping a second copy that can drift.