feat: add gitlawb-compatible commands — cert, ipfs, mcp, sync, name, star, changelog, whoami

- cert: list/show signed ref-update certificates
- ipfs: list/get IPFS pins
- mcp: serve MCP server placeholder (use gitant-mcp)
- sync: trigger/status peer node sync
- name: register/resolve/lookup/available/register-did/resolve-did on Base L2
- star: add/remove/count repo stars
- changelog: unified activity feed
- whoami: print current DID identity

Author: Patel230