Let cancel terminate auxiliary build phases instead of blocking the slot

Cancelling a build only killed the running build process; the synchronous
preparation phases — most notably a multi-minute Docker image build, but also
the Gradle-volume preparation — ran to completion and kept the concurrency
slot occupied, so the next queued build stayed PENDING for a long time.
Build runners now report every auxiliary process through an onAuxProcess sink
(GitCommandRunner gained an onProcess hook), and the executor registers them
like the build process, so cancellation terminates whatever is currently
running. Measured on vm4006: cancel to next-build-running is ~3s in the
normal case; the unit test covers the aux-phase case.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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

@@ -23,27 +23,39 @@ class GitCommandException(
 
 @Component
 class GitCommandRunner {
+    /** [onProcess] receives the started process, so callers can terminate it early (build cancellation). */
     fun run(
         command: List<String>,
         workingDir: Path,
         environment: Map<String, String> = emptyMap(),
+        onProcess: (Process) -> Unit = {},
     ): 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()
+        onProcess(process)
+        // stderr is drained concurrently so neither pipe buffer can block the process;
+        // reads tolerate closed streams, e.g. when onProcess terminated the process early
+        val stderr = CompletableFuture.supplyAsync { process.errorStream.readSafely() }
+        val stdout = process.inputStream.readSafely()
         val exitCode = process.waitFor()
         return GitCommandResult(exitCode, stdout, stderr.get())
     }
 
+    private fun java.io.InputStream.readSafely(): String =
+        try {
+            bufferedReader().readText()
+        } catch (_: java.io.IOException) {
+            ""
+        }
+
     fun runOrThrow(
         command: List<String>,
         workingDir: Path,
         environment: Map<String, String> = emptyMap(),
+        onProcess: (Process) -> Unit = {},
     ): GitCommandResult {
-        val result = run(command, workingDir, environment)
+        val result = run(command, workingDir, environment, onProcess)
         if (!result.isSuccess) {
             throw GitCommandException(command, result)
         }