implemented 04-build-executor.md incl. concurrency amendment: async builds in per-branch worktrees, builds.maxConcurrent, cancellation, live logs; fix .gitignore build/ rule that silently excluded the de.hoennig.gittally.build package (also recovers the step-01 domain files)

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

src/main/kotlin/de/hoennig/gittally/build/ArtifactKeys.kt

@@ -0,0 +1,27 @@
+package de.hoennig.gittally.build
+
+import java.security.MessageDigest
+import java.time.Instant
+
+/**
+ * Legacy-compatible artifact key naming: sanitized name plus a 12-char SHA-256 prefix,
+ * so keys are filesystem- and URL-safe but still unique for branch names that
+ * sanitize to the same string.
+ */
+object ArtifactKeys {
+    fun branchKey(branch: String): String = "${sanitize(branch)}-${sha256Prefix(branch)}"
+
+    fun buildKey(
+        branch: String,
+        startedAt: Instant,
+    ): String = "${branchKey(branch)}-${sanitize(startedAt.toString())}-${sha256Prefix("$branch\t$startedAt")}"
+
+    private fun sanitize(value: String): String = value.replace(Regex("[^A-Za-z0-9._-]"), "_")
+
+    private fun sha256Prefix(value: String): String =
+        MessageDigest
+            .getInstance("SHA-256")
+            .digest(value.toByteArray())
+            .joinToString("") { "%02x".format(it) }
+            .take(12)
+}