Remove plan status guardrails

[nwang783]

Summary

• remove backend transition guardrails so agents can set any non-empty plan status
• add a generic gsync plan status command while keeping the existing review/merged shortcuts
• keep custom non-terminal statuses visible in sync and dashboard active-plan surfaces

Testing

• npm test
• npm --prefix dashboard test
• npm run dev:seed
• npm run dev:cli -- plan status X4OpsrMK35oygT3bxRAw blocked-on-design
• npm run dev:cli -- sync

Closes #10