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/test/kotlin/de/hoennig/gittally/git/GitCommandRunnerTest.kt

@@ -5,6 +5,7 @@ import io.kotest.core.spec.style.FunSpec
 import io.kotest.matchers.booleans.shouldBeFalse
 import io.kotest.matchers.booleans.shouldBeTrue
 import io.kotest.matchers.collections.shouldContainExactly
+import io.kotest.matchers.nulls.shouldNotBeNull
 import io.kotest.matchers.shouldBe
 import io.kotest.matchers.string.shouldContain
 import java.nio.file.Files
@@ -30,6 +31,15 @@ class GitCommandRunnerTest : FunSpec() {
             result.stdout.trim() shouldBe tempDir.toRealPath().toString()
         }
 
+        test("hands the started process to onProcess, so callers can terminate it early") {
+            var seen: Process? = null
+
+            val result = runner.run(listOf("sh", "-c", "sleep 30"), tempDir, onProcess = { seen = it.also(Process::destroy) })
+
+            result.isSuccess.shouldBeFalse()
+            seen.shouldNotBeNull().isAlive.shouldBeFalse()
+        }
+
         test("passes extra environment variables") {
             val result = runner.run(listOf("sh", "-c", "echo \"\$GITTALLY_TEST_VAR\""), tempDir, mapOf("GITTALLY_TEST_VAR" to "hello"))