Skill Reminder News and Invocation Log — discoverability system

Three-layer skill discoverability: (A) on-error hint: related skills shown on non-zero exit; (B) on-success tip: unseen skills surfaced for successful commands; (C) invocation log: runtime/skill_invocations.jsonl tracks every skill read. Skill sync diff notification deferred. Components: CommandExitHelper (PrintSkillHintOnError/OnSuccess, skill_seen.jsonl), SkillCommand (AppendSkillInvocationLog, skill_invocations.jsonl).

Steps