Validate workspace color during cmux.json decode (#2112)

Reject invalid color values at parse time with a clear error message
instead of silently ignoring them. Colors are normalized to #RRGGBB
via WorkspaceTabColorSettings.normalizedHex during decode.

Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>

Sources/CmuxConfig.swift

@@ -95,6 +95,33 @@ struct CmuxWorkspaceDefinition: Codable, Sendable {
     var cwd: String?
     var color: String?
     var layout: CmuxLayoutNode?
+
+    init(name: String? = nil, cwd: String? = nil, color: String? = nil, layout: CmuxLayoutNode? = nil) {
+        self.name = name
+        self.cwd = cwd
+        self.color = color
+        self.layout = layout
+    }
+
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        name = try container.decodeIfPresent(String.self, forKey: .name)
+        cwd = try container.decodeIfPresent(String.self, forKey: .cwd)
+        layout = try container.decodeIfPresent(CmuxLayoutNode.self, forKey: .layout)
+
+        if let rawColor = try container.decodeIfPresent(String.self, forKey: .color) {
+            guard let normalized = WorkspaceTabColorSettings.normalizedHex(rawColor) else {
+                throw DecodingError.dataCorruptedError(
+                    forKey: .color,
+                    in: container,
+                    debugDescription: "Invalid color \"\(rawColor)\". Expected 6-digit hex format: #RRGGBB"
+                )
+            }
+            color = normalized
+        } else {
+            color = nil
+        }
+    }
 }
 
 indirect enum CmuxLayoutNode: Codable, Sendable {