Audit the license headers of all source files #154

A file written in this fork carries the Apache 2.0 header alone, without a copyright line, as open-java-format-idea-plugin/src/test/java/com/palantir/javaformat/intellij/PalantirJavaFormatConfigurableTest.java does and as CLAUDE.md now says. Files inherited from google-java-format and palantir-java-format keep their original Google or Palantir headers.

The tree does not follow that rule yet. Of the Java files, 94 start with a Palantir copyright and 58 with a Google one, and some of the Palantir ones sit on files this fork wrote, with the current year (JavaCommentsHelperTest.java, for one). #150 corrected the four files it adds.

• List every file whose header does not match its origin: fork-written files with a Palantir or Google header, and files without a header. The git history tells an inherited file from a new one.
• Put the plain Apache header on the fork-written files.
• Make the build fail on a new file with the wrong header: checkstyle's RegexpHeader in config/checkstyle, or a Spotless licenseHeader step, whichever fits the build.