NEO-48: revert WhenWritingNull on skill-progression grant API

Keep omit-null reasonCode only on new perk-state responses; restoring
the existing grant endpoint wire format (reasonCode: null on success).

bruno/neon-sprawl-server/skill-progression/Post skill progression grant.bru

@@ -8,6 +8,7 @@ docs {
   NEO-38: single skill XP grant + allowlist + level-up metadata; see server README.
   NEO-40: same contract; server adds comment-only telemetry hook markers (no request/response change).
   NEO-47: on level-up, server re-evaluates mastery perk unlocks internally (no JSON change until NEO-48 perk-state HTTP).
+  Success responses include reasonCode: null (stable since NEO-38); perk-state POST omits reasonCode on success instead.
   E2.M2 tracking row in docs/decomposition/modules/documentation_and_implementation_alignment.md lists NEO-40 landed with plan + manual QA links.
 }
 
@@ -32,6 +33,7 @@ tests {
     const body = res.getBody();
     expect(body.schemaVersion).to.equal(1);
     expect(body.granted).to.equal(true);
+    expect(body.reasonCode).to.equal(null);
     expect(body.progression).to.be.an("object");
     expect(body.progression.schemaVersion).to.equal(1);
     expect(body.progression.playerId).to.equal("dev-local-1");

server/NeonSprawl.Server.Tests/Game/Skills/SkillProgressionGrantApiTests.cs

@@ -1,6 +1,7 @@
 using System.Linq;
 using System.Net;
 using System.Net.Http.Json;
+using System.Text.Json;
 using NeonSprawl.Server.Game.Skills;
 using Xunit;
 
@@ -120,6 +121,26 @@ public sealed class SkillProgressionGrantApiTests
         Assert.Equal(SkillProgressionSnapshotApi.ReasonInvalidAmount, envelope.ReasonCode);
     }
 
+    [Fact]
+    public async Task PostSkillProgression_ShouldSerializeReasonCodeNull_OnSuccess()
+    {
+        // Arrange
+        await using var factory = new InMemoryWebApplicationFactory();
+        var client = factory.CreateClient();
+
+        // Act
+        var response = await client.PostAsJsonAsync(
+            "/game/players/dev-local-1/skill-progression",
+            ValidGrant("salvage", amount: 10));
+
+        // Assert
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var json = await response.Content.ReadAsStringAsync();
+        using var doc = JsonDocument.Parse(json);
+        Assert.True(doc.RootElement.TryGetProperty("reasonCode", out var reasonCode));
+        Assert.Equal(JsonValueKind.Null, reasonCode.ValueKind);
+    }
+
     [Fact]
     public async Task PostSkillProgression_ShouldApplyGrant_AndGetMatches_WhenTrainerAllowedForRefine()
     {

server/NeonSprawl.Server/Game/Skills/SkillProgressionGrantDtos.cs

@@ -34,7 +34,6 @@ public sealed class SkillProgressionGrantResponse
 
     /// <remarks>Set when <see cref="Granted"/> is <c>false</c>; stable snake_case values (see README).</remarks>
     [JsonPropertyName("reasonCode")]
-    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
     public string? ReasonCode { get; init; }
 
     [JsonPropertyName("progression")]