feat(workspace): add marks support to workspaces

Workspaces can now have marks just like containers. This enables the
pattern of marking a workspace with [con_id=X] and then using
"move container to mark" to reparent containers to workspaces.

Changes:
- Add marks list to sway_workspace struct
- Add workspace_has_mark, workspace_add_mark, workspace_clear_marks,
  workspace_find_and_unmark, workspace_find_mark functions
- Update mark command to work on workspaces when targeted via criteria
- Update unmark command to support workspaces
- Update move.c to find marks on workspaces for "move to mark"
- Update criteria con_mark matching to also check workspaces
- Add workspace marks to IPC JSON output

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

include/sway/tree/workspace.h

@@ -48,6 +48,7 @@ struct sway_workspace {
 	list_t *floating;           // struct sway_container
 	list_t *tiling;             // struct sway_container
 	list_t *output_priority;
+	list_t *marks;              // char *
 	bool urgent;
 
 	struct sway_workspace_state current;
@@ -157,4 +158,14 @@ size_t workspace_num_sticky_containers(struct sway_workspace *ws);
  */
 void workspace_squash(struct sway_workspace *workspace);
 
+bool workspace_has_mark(struct sway_workspace *ws, char *mark);
+
+void workspace_add_mark(struct sway_workspace *ws, char *mark);
+
+void workspace_clear_marks(struct sway_workspace *ws);
+
+bool workspace_find_and_unmark(char *mark);
+
+struct sway_workspace *workspace_find_mark(char *mark);
+
 #endif