Compile Git for host and Android target platforms

- Rename CrossCompileGitForAndroid.sh to CompileGitForAllTargetPlatforms.sh and support host, Android-only, and all-target modes.
- Compile a development-host Git binary at build/host-git/libgit.so for JVM tests.
- Route level and sandbox tests through GITHUG_TEST_GIT_BINARY so they exercise the compiled host Git rather than the system Git fallback.
- Cross-compile packaged Git binaries for arm64-v8a, armeabi-v7a, x86, and x86_64 Android targets.
- Remove the temporary device-bound Pull rebase instrumentation path from the previous debugging attempt.

app/src/test/java/solutions/tretter/githugandroid/GitSandboxEngineTest.kt

@@ -152,7 +152,7 @@ class GitSandboxEngineTest {
 
     @Test
     fun nativeCompletionUsesCurrentDirectoryIncludingGitMetadata() {
-        val git = File("/usr/bin/git")
+        val git = testGitBinary()
         assumeTrue(git.exists() && git.canExecute())
         val root = Files.createTempDirectory("githug-completion").toFile()
         try {
@@ -177,7 +177,7 @@ class GitSandboxEngineTest {
 
     @Test
     fun nativePushLevelHasDivergedStatusAndRebasesCleanly() {
-        val git = File("/usr/bin/git")
+        val git = testGitBinary()
         assumeTrue(git.exists() && git.canExecute())
         val root = Files.createTempDirectory("githug-push-level").toFile()
         try {
@@ -221,4 +221,22 @@ class GitSandboxEngineTest {
         assertTrue(output.isEmpty())
         assertFalse(updatedRepo.files.any { it.name == "deleteme.txt" })
     }
+
+    private fun testGitBinary(): File {
+        System.getenv("GITHUG_TEST_GIT_BINARY")
+            ?.takeIf { it.isNotBlank() }
+            ?.let { File(it) }
+            ?.takeIf { it.exists() && it.canExecute() }
+            ?.let { return it }
+
+        val repoHostGit = File(repoRoot(), "build/host-git/libgit.so")
+        if (repoHostGit.exists() && repoHostGit.canExecute()) return repoHostGit
+
+        return File("/usr/bin/git")
+    }
+
+    private fun repoRoot(): File {
+        val userDir = File(System.getProperty("user.dir") ?: ".")
+        return if (File(userDir, "app/build.gradle.kts").exists()) userDir else userDir.parentFile ?: userDir
+    }
 }