Rename ChatAgent to GaiaAgent across codebase

[kovtcharov]

Summary

The main consumer agent should be called GaiaAgent, not ChatAgent. "Chat" undersells the capability — it is the core product agent, not just a chatbot.

Scope

• Rename class: ChatAgent → GaiaAgent in src/gaia/agents/chat/agent.py
• Rename directory: src/gaia/agents/chat/ → src/gaia/agents/gaia/ (or keep chat/ as alias)
• Update all imports and references across codebase
• Update CLI: gaia chat → keep as alias, add gaia agent
• Update documentation references
• Update API server agent registry
• Update MCP bridge registration

Acceptance Criteria

• Class renamed to GaiaAgent
• All imports updated
• CLI still works (gaia chat as alias for backwards compat)
• Documentation updated
• Tests pass