Show the command a build actually runs on its artifact page

The artifact page read `branches.<branch>.buildCommand` from the primary
config only, so for a named build on a branch with its own config it showed
a command that build never ran — on vm4006 it showed the host config's
command for a build that ran the branch definition's `pitestFull`.

Resolving "what does this build run" now has one implementation,
`GitTallyConfig.buildSettings(branch, build)`: the branch entry with the
build definition's overrides applied last. The executor uses it, and the
page resolves it against the branch layer committed at the build's own
commit — the same inputs the executor had.

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

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

@@ -21,6 +21,22 @@ data class GitTallyConfig(
     /** The configured [buildDefinitions] plus the implicit `default` build unless overridden. */
     fun effectiveBuildDefinitions(): Map<String, BuildDefinition> =
         mapOf(BuildDefinition.DEFAULT to BuildDefinition(onPush = true)) + buildDefinitions
+
+    /**
+     * The settings one build of [build] on [branch] runs with: the branch entry (falling
+     * back to `branches.default`) with the build definition's overrides applied last.
+     * A build name without a definition — a job removed from the config since the run —
+     * falls back to the plain branch settings.
+     * This must stay the single answer to "what does this build run", used by the
+     * executor and by everything that displays it.
+     */
+    fun buildSettings(
+        branch: String,
+        build: String,
+    ): BranchConfig {
+        val branchConfig = branches[branch] ?: branches["default"] ?: BranchConfig()
+        return effectiveBuildDefinitions()[build]?.applyTo(branchConfig) ?: branchConfig
+    }
 }
 
 data class ServerConfig(