feat(dolt): add federation prototype tests

Add integration tests validating Dolt APIs needed for peer-to-peer
federation between Gas Towns:

- TestFederationDatabaseIsolation: verify separate towns have isolated DBs
- TestFederationVersionControlAPIs: branch, checkout, merge, commit
- TestFederationRemoteConfiguration: AddRemote accepts dolthub:// URLs
- TestFederationHistoryQueries: commit history, dolt_log, AS OF

Key finding: Production federation uses dolt sql-server remotesapi
(port 8080) for direct peer-to-peer push/pull - no DoltHub required.

Also fixes concurrent_test.go Merge() return value handling.

Refs: bd-wkumz (Federation epic), hq-ew1mbr.7 (prototype task)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/dolt/concurrent_test.go

@@ -459,13 +459,13 @@ func TestBranchPerAgentMergeRace(t *testing.T) {
 	}
 
 	// First merge should succeed
-	_, err1 := store.Merge(ctx, "agent-1")
+	conflicts1, err1 := store.Merge(ctx, "agent-1")
 
 	// Second merge may conflict (both modified same row)
-	_, err2 := store.Merge(ctx, "agent-2")
+	conflicts2, err2 := store.Merge(ctx, "agent-2")
 
-	t.Logf("Merge agent-1 result: %v", err1)
-	t.Logf("Merge agent-2 result: %v", err2)
+	t.Logf("Merge agent-1 result: err=%v conflicts=%d", err1, len(conflicts1))
+	t.Logf("Merge agent-2 result: err=%v conflicts=%d", err2, len(conflicts2))
 
 	// At least one should succeed
 	if err1 != nil && err2 != nil {