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/agents.py

@@ -9,8 +9,7 @@ AGENT_ALIASES: dict[str, str] = {
     "claude": "claude",
     "claude-code": "claude",
     "codex": "codex",
-    "opencode": "opencode",
-    "*": "*",
+    "agents": "agents",
 }
 
 AGENT_CHOICES = sorted({*ALL_TARGETS, *AGENT_ALIASES.keys()})