Add Phase 1 RFCs: abstractions, env-spec, MCP support, and unified actions

- RFC 000: Project phases and approach
- RFC 001: Core abstractions (Environment, Agent, Task, Dataset)
- RFC 002: Environment specification (reset, step, state APIs)
- RFC 003: MCP integration for tools (remote + local, CodeAct support)
- RFC 004: Unified action interface for all environment types

This PR reorganizes and expands our RFC structure:
- Renamed old RFC 002 → RFC 004
- Renamed old RFC 001 → RFC 002
- Added new RFC 001 (abstractions) and RFC 003 (MCP)
- Updated RFC 000 with review feedback
- Harmonized language and cross-references across all RFCs

Author: Darktex