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/build/BuildRunner.kt

@@ -10,6 +10,9 @@ import java.nio.file.Path
  * which owns log streaming and process-tree termination.
  * [repoDir] and [branchConfig] let implementations derive per-repository resources
  * and per-branch settings (step 11: Docker runtime).
+ * Implementations report every auxiliary process they run before the build command
+ * (e.g. a Docker image build) via [onAuxProcess], so a cancellation can terminate
+ * long preparation phases instead of blocking the build slot until they finish.
  */
 interface BuildRunner {
     fun start(
@@ -18,6 +21,7 @@ interface BuildRunner {
         environment: Map<String, String>,
         repoDir: Path = workingDir,
         branchConfig: BranchConfig = BranchConfig(),
+        onAuxProcess: (Process) -> Unit = {},
     ): Process
 }
 
@@ -29,6 +33,7 @@ class ProcessBuildRunner : BuildRunner {
         environment: Map<String, String>,
         repoDir: Path,
         branchConfig: BranchConfig,
+        onAuxProcess: (Process) -> Unit,
     ): Process {
         val processBuilder = ProcessBuilder("bash", "-c", command)
         processBuilder.directory(workingDir.toFile())
@@ -53,8 +58,9 @@ class DispatchingBuildRunner(
         environment: Map<String, String>,
         repoDir: Path,
         branchConfig: BranchConfig,
+        onAuxProcess: (Process) -> Unit,
     ): Process {
         val runner = if (branchConfig.docker.enabled) dockerBuildRunner else processBuildRunner
-        return runner.start(command, workingDir, environment, repoDir, branchConfig)
+        return runner.start(command, workingDir, environment, repoDir, branchConfig, onAuxProcess)
     }
 }