xint-mcp-tiers-must-match-credential-requirements

MCP tool exposure must be gated by credential availability: read_only policy for xAI-only tools (Phase 1), separate tier for X API v2 read tools (Phase 2), engagement tier with OAuth requirement for write operations (Phase 2b). Registering OAuth-gated tools under read_only policy creates runtime failures when unauthenticated write calls execute. Validate credential presence before exposing each MCP tier.