✨ feat(mergelane): serialized lane core with durable front record and pinned merge

changelog.d/added-mergelane-core.md

@@ -0,0 +1 @@
+- mergelane: core of the serialized merge lane for `hive-serialized` repositories ([#10888](https://github.com/hivecommons/hive/issues/10888), part of [#10884](https://github.com/hivecommons/hive/issues/10884)). A new `pkg/mergelane` keeps a durable, fenced front record per (repo, target branch) under `/data/mergelane/`, orders pull requests oldest-eligible first, times out a stuck front (60 minutes by default, per-repo override, restarted on every update) and drives the front pull request through compare-based staleness detection, a merge update pinned to the evaluated head, required checks on the exact head (unknown or empty set means no merge), a live final re-check, a head-pinned merge and a post-merge parent check that alerts when the merge did not land on the validated tip. Every front entry and exit, update, merge and refusal is audited. The lane is not yet called by any merge path, so nothing changes for `direct` repositories.

src/pkg/config/repo_merge_strategy_test.go

@@ -76,9 +76,11 @@ func TestInvalidMergeStrategyFailsValidationAndStopsAutoMerge(t *testing.T) {
 	cfg.ACMMLevel = &level
 	cfg.Project.RepoPolicies = []RepoPolicy{{Repo: "console", MergeStrategy: "hive-serialised"}}
 
-	err := cfg.Validate()
+	// No agents on purpose: RequireAgents=false so the merge_strategy check is
+	// the first thing that can fail.
+	err := cfg.ValidateWithOptions(ValidateOptions{})
 	if err == nil || !strings.Contains(err.Error(), "console") || !strings.Contains(err.Error(), "hive-serialised") {
-		t.Fatalf("Validate() = %v, want error naming repo and value", err)
+		t.Fatalf("ValidateWithOptions() = %v, want error naming repo and value", err)
 	}
 	if cfg.RepoAutoMergeEnabled("console") {
 		t.Error("auto-merge must be off for a repo with an invalid merge_strategy")