Add canonical previous-session artifact source parsing and validation

internal/artifacts/artifact_resolver_test.go

@@ -45,6 +45,58 @@ func TestNormalizeSessionArtifactSource(t *testing.T) {
 	}
 }
 
+func TestPreviousSessionArtifactSourceHelpers(t *testing.T) {
+	tests := []struct {
+		name      string
+		source    string
+		wantName  string
+		wantMatch bool
+	}{
+		{
+			name:      "valid",
+			source:    "narratio.previous_session.artifact.session_recap",
+			wantName:  "session_recap",
+			wantMatch: true,
+		},
+		{
+			name:      "valid with surrounding whitespace",
+			source:    "  narratio.previous_session.artifact.quest_log  ",
+			wantName:  "quest_log",
+			wantMatch: true,
+		},
+		{
+			name:      "missing name",
+			source:    "narratio.previous_session.artifact.",
+			wantMatch: false,
+		},
+		{
+			name:      "invalid key characters",
+			source:    "narratio.previous_session.artifact.session-recap",
+			wantMatch: false,
+		},
+		{
+			name:      "wrong prefix",
+			source:    "narratio.previous.artifact.session_recap",
+			wantMatch: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			name, ok := PreviousSessionArtifactName(tt.source)
+			if ok != tt.wantMatch {
+				t.Fatalf("PreviousSessionArtifactName(%q) ok = %t, want %t", tt.source, ok, tt.wantMatch)
+			}
+			if name != tt.wantName {
+				t.Fatalf("PreviousSessionArtifactName(%q) name = %q, want %q", tt.source, name, tt.wantName)
+			}
+			if got := IsPreviousSessionArtifactSource(tt.source); got != tt.wantMatch {
+				t.Fatalf("IsPreviousSessionArtifactSource(%q) = %t, want %t", tt.source, got, tt.wantMatch)
+			}
+		})
+	}
+}
+
 func TestResolveSessionArtifactPrefersManifestOutput(t *testing.T) {
 	workspace := t.TempDir()
 	paths := buildSessionPaths(workspace, "campaign", "session")