Move the concurrency limit to executor.maxConcurrent

builds.maxConcurrent mixed an execution setting into the build
definitions as a reserved key. The limit now lives in the new executor
section (pinned like the builds section, enforced for all builds
regardless of trigger), default 1, without a compatibility alias — a
leftover builds.maxConcurrent key is rejected as an invalid build
definition. Recorded as a follow-up in ADR 0007.

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

src/main/kotlin/de/hoennig/gittally/config/GitTallyConfig.kt

@@ -1,18 +1,21 @@
 package de.hoennig.gittally.config
 
+import com.fasterxml.jackson.annotation.JsonProperty
+
 data class GitTallyConfig(
     val server: ServerConfig = ServerConfig(),
     val git: GitConfig = GitConfig(),
     val gitea: GiteaConfig = GiteaConfig(),
-    val builds: BuildsConfig = BuildsConfig(),
+    val executor: ExecutorConfig = ExecutorConfig(),
     val artifacts: ArtifactsConfig = ArtifactsConfig(),
     val watcher: WatcherConfig = WatcherConfig(),
     val branches: Map<String, BranchConfig> = mapOf("default" to BranchConfig()),
     /**
-     * Named build definitions (jobs) over the branches (ADR 0007). The implicit
-     * [BuildDefinition.DEFAULT] build (`onPush` over all branches) applies unless this
-     * map overrides it; see [effectiveBuildDefinitions].
+     * Named build definitions (jobs) over the branches (ADR 0007), the YAML `builds`
+     * section. The implicit [BuildDefinition.DEFAULT] build (`onPush` over all
+     * branches) applies unless this map overrides it; see [effectiveBuildDefinitions].
      */
+    @JsonProperty("builds")
     val buildDefinitions: Map<String, BuildDefinition> = emptyMap(),
 ) {
     /** The configured [buildDefinitions] plus the implicit `default` build unless overridden. */
@@ -78,11 +81,6 @@ data class GiteaConfig(
     val statusContext: String = "GitTally",
 )
 
-data class BuildsConfig(
-    /** How many branches may build at the same time; at most one build per branch regardless. */
-    val maxConcurrent: Int = 1,
-)
-
 data class ArtifactsConfig(
     val retentionPerBranch: Int = 3,
     /**
@@ -156,6 +154,12 @@ data class DockerConfig(
     val env: Map<String, String> = emptyMap(),
 )
 
+/** Build execution settings, enforced by the executor for all builds regardless of their trigger. */
+data class ExecutorConfig(
+    /** How many builds may run at the same time; at most one build per branch runs regardless. */
+    val maxConcurrent: Int = 1,
+)
+
 /**
  * Deprecated per-branch schedule (pre-ADR-0007), kept for compatibility: mapped to a
  * daily rebuild of the branch's own pool with its regular command. New configurations