feat(skiff): replace opencode target with agents standard dir

- targets: cursor, claude, codex, agents (~/.agents/skills, .agents/skills)
- agents dir covers OMP native discovery; drop omp/opencode targets
- project-level agents/cursor/codex share .agents/skills via idempotent symlinks
- migrate local state: remove ~/.config/opencode/skills links, reinstall into ~/.agents/skills

skiff/cli.py

@@ -1655,7 +1655,7 @@ def _add_common_flags(parser: argparse.ArgumentParser) -> None:
         nargs="+",
         action="append",
         metavar="AGENT",
-        help="目标 agent（cursor、claude、claude-code、codex、opencode、*）",
+        help="目标 agent（cursor、claude、claude-code、codex、agents、*）",
     )
     parser.add_argument(
         "-g",