Add test coverage for .htm files and fix README

Add tests to verify both .html and .htm files are discovered when
sourceDocuments is auto-populated from sourceDir.

- Maven plugin: Test verifies 3 files (.htm, .html, nested .htm)
- Gradle plugin: Test verifies same file discovery pattern
- Both tests confirm recursive directory scanning works for .htm
- Fix Maven README: Use correct <sourceDocument> element name
- All tests pass with no regressions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ascheman

htmlSanityCheck-gradle-plugin/src/test/groovy/org/aim42/htmlsanitycheck/gradle/HtmlSanityCheckTaskSpec.groovy

@@ -62,4 +62,32 @@ class HtmlSanityCheckTaskSpec extends HtmlSanityCheckBaseSpec {
         e.message.contains("Your build configuration included 'failOnErrors=true', and 1 error(s) were found on all checked pages.")
     }
 
+    def "should include .htm files when auto-populating sourceDocuments"() {
+        given:
+        // Create .htm file
+        def htmFile = new File(sourceDir, "document.htm")
+        htmFile << VALID_HTML
+
+        // Create .html file
+        htmlFile << VALID_HTML
+
+        // Create subdirectory with .htm file
+        def subDir = new File(sourceDir, "docs")
+        subDir.mkdirs()
+        def nestedHtmFile = new File(subDir, "nested.htm")
+        nestedHtmFile << VALID_HTML
+
+        when:
+        task.setSourceDir(testProjectDir.root)
+
+        then:
+        task.sourceDocuments != null
+        // Verify that sourceDocuments includes both .html and .htm files
+        def fileNames = task.sourceDocuments.files.collect { it.name }
+        fileNames.contains("test.html")
+        fileNames.contains("document.htm")
+        fileNames.contains("nested.htm")
+        task.sourceDocuments.files.size() == 3
+    }
+
 }

htmlSanityCheck-maven-plugin/README.adoc

@@ -41,7 +41,7 @@ Use the following snippet inside a Maven build file:
     </executions>
     <configuration>
         <sourceDocuments>
-            <file>src/file-to-test.html</file> <2>
+            <sourceDocument>src/file-to-test.html</sourceDocument> <2>
         </sourceDocuments>
         <sourceDir>src</sourceDir> <3>
     </configuration>

htmlSanityCheck-maven-plugin/src/test/java/org/aim42/htmlsanitycheck/maven/HtmlSanityCheckMojoTest.java

@@ -226,6 +226,55 @@ void executeWithOnlySourceDir_ShouldSucceed() throws IOException, MojoExecutionE
         deleteDirectory(resultDir.toFile());
     }
 
+    @Test
+    void executeWithOnlySourceDir_ShouldIncludeHtmFiles() throws IOException, MojoExecutionException {
+        // Setup: Create temp directories
+        Path junitDir = Files.createTempDirectory("MojoJunit");
+        Path resultDir = Files.createTempDirectory("MojoResult");
+        Path sourceDir = Files.createTempDirectory("MojoSource");
+
+        // Create .htm file in root
+        File rootHtmFile = new File(sourceDir.toFile(), "document.htm");
+        Files.write(rootHtmFile.toPath(), VALID_HTML.getBytes(StandardCharsets.UTF_8));
+
+        // Create .html file for comparison
+        File rootHtmlFile = new File(sourceDir.toFile(), "page.html");
+        Files.write(rootHtmlFile.toPath(), VALID_HTML.getBytes(StandardCharsets.UTF_8));
+
+        // Create subdirectory with .htm file
+        File subDir = new File(sourceDir.toFile(), "docs");
+        boolean mkdirSuccess = subDir.mkdirs();
+        Assertions.assertThat(mkdirSuccess).isTrue();
+        File nestedHtmFile = new File(subDir, "nested.htm");
+        Files.write(nestedHtmFile.toPath(), VALID_HTML.getBytes(StandardCharsets.UTF_8));
+
+        // Create Mojo and set only sourceDir field
+        HtmlSanityCheckMojo mojo = new TestableHtmlSanityCheckMojo(
+                sourceDir.toFile(),
+                null, // sourceDocuments explicitly NOT set
+                resultDir.toFile(),
+                junitDir.toFile()
+        );
+
+        // Get the configuration to verify sourceDocuments includes .htm files
+        Configuration config = mojo.setupConfiguration();
+
+        // Verify that both .html and .htm files are discovered
+        Assertions.assertThat(config.getSourceDocuments()).isNotNull();
+        Assertions.assertThat(config.getSourceDocuments()).hasSize(3);
+        Assertions.assertThat(config.getSourceDocuments())
+                .extracting(File::getName)
+                .containsExactlyInAnyOrder("document.htm", "page.html", "nested.htm");
+
+        // Execute should succeed with both file types
+        mojo.execute();
+
+        // Clean up
+        deleteDirectory(sourceDir.toFile());
+        deleteDirectory(junitDir.toFile());
+        deleteDirectory(resultDir.toFile());
+    }
+
     /**
      * Helper class to allow setting private fields for testing
      */