fix: restore ArtifactMessages.cs missed by case-insensitive gitignore

The `artifacts/` rule in .gitignore matched both the root build-output
directory and the `src/ARCP/Messages/Artifacts/` source folder on
macOS's case-insensitive filesystem, so ArtifactPut, ArtifactRef,
ArtifactFetch, and ArtifactRelease were never tracked. Linux CI
cloned the tree without them and failed to build samples/Handoff and
ARCP.IntegrationTests with CS0246. Scope the rule to /artifacts/ and
add the missing file.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nficano

.gitignore

@@ -5,7 +5,10 @@ obj/
 *.userosscache
 *.sln.docstates
 [Tt]est[Rr]esult*/
-artifacts/
+# Leading slash scopes the rule to the repo-root build output dir so it
+# doesn't also match source folders like src/ARCP/Messages/Artifacts/
+# under macOS's case-insensitive filesystem.
+/artifacts/
 
 # Coverage
 coverage/

src/ARCP/Messages/Artifacts/ArtifactMessages.cs

@@ -0,0 +1,42 @@
+using ARCP.Envelope;
+
+namespace ARCP.Messages.Artifacts;
+
+/// <summary>§16.2 upload an artifact (inline base64 in v0.1; sidecar deferred).</summary>
+public sealed record ArtifactPut(
+    string MediaType,
+    Ids.ArtifactId? ArtifactId = null,
+    string? Data = null,
+    string? Encoding = null,
+    int? TtlSeconds = null) : MessageType
+{
+    /// <inheritdoc />
+    public override string WireType => "artifact.put";
+}
+
+/// <summary>§16.2 fetch an artifact by id.</summary>
+public sealed record ArtifactFetch(Ids.ArtifactId ArtifactId) : MessageType
+{
+    /// <inheritdoc />
+    public override string WireType => "artifact.fetch";
+}
+
+/// <summary>§16.1 canonical artifact reference.</summary>
+public sealed record ArtifactRef(
+    Ids.ArtifactId ArtifactId,
+    string Uri,
+    string MediaType,
+    long Size,
+    string? Sha256 = null,
+    DateTimeOffset? ExpiresAt = null) : MessageType
+{
+    /// <inheritdoc />
+    public override string WireType => "artifact.ref";
+}
+
+/// <summary>§16.2 release a fetched artifact.</summary>
+public sealed record ArtifactRelease(Ids.ArtifactId ArtifactId) : MessageType
+{
+    /// <inheritdoc />
+    public override string WireType => "artifact.release";
+}