implemented 02-git-gateway.md: added Git access layer with credentials bridge, duration parser, and extensive tests

src/main/kotlin/de/hoennig/gittally/git/GitCommandRunner.kt

@@ -0,0 +1,52 @@
+package de.hoennig.gittally.git
+
+import org.springframework.stereotype.Component
+import java.nio.file.Path
+import java.util.concurrent.CompletableFuture
+
+data class GitCommandResult(
+    val exitCode: Int,
+    val stdout: String,
+    val stderr: String,
+) {
+    val isSuccess: Boolean get() = exitCode == 0
+
+    fun lines(): List<String> = stdout.lines().map { it.trim() }.filter { it.isNotEmpty() }
+}
+
+class GitCommandException(
+    command: List<String>,
+    result: GitCommandResult,
+) : RuntimeException(
+        "Command failed with exit code ${result.exitCode}: ${command.joinToString(" ")}\n${result.stderr.trim()}",
+    )
+
+@Component
+class GitCommandRunner {
+    fun run(
+        command: List<String>,
+        workingDir: Path,
+        environment: Map<String, String> = emptyMap(),
+    ): GitCommandResult {
+        val builder = ProcessBuilder(command).directory(workingDir.toFile())
+        builder.environment().putAll(environment)
+        val process = builder.start()
+        // stderr is drained concurrently so neither pipe buffer can block the process
+        val stderr = CompletableFuture.supplyAsync { process.errorStream.bufferedReader().readText() }
+        val stdout = process.inputStream.bufferedReader().readText()
+        val exitCode = process.waitFor()
+        return GitCommandResult(exitCode, stdout, stderr.get())
+    }
+
+    fun runOrThrow(
+        command: List<String>,
+        workingDir: Path,
+        environment: Map<String, String> = emptyMap(),
+    ): GitCommandResult {
+        val result = run(command, workingDir, environment)
+        if (!result.isSuccess) {
+            throw GitCommandException(command, result)
+        }
+        return result
+    }
+}