Add Matrix support with platform adapters
- adapters/base.py: Platform-agnostic abstractions (PlatformMessage, PlatformAdapter)
- adapters/discord_adapter.py: Discord adapter using discord.py
- adapters/matrix_adapter.py: Matrix adapter using matrix-nio
- bot_core.py: Shared message handling logic (vision, chat)
- matrix_bot.py: Entry point for Matrix bot
- requirements.txt: Added matrix-nio[e2e]