docs: add module-level documentation to core modules

Add comprehensive //! module docs to high-priority files:
- fluent-core/neo4j_client.rs: Neo4j client with vector embeddings
- fluent-core/auth.rs: Authentication and credential management
- fluent-core/config.rs: Configuration management (YAML/JSON/TOML)
- fluent-agent/orchestrator.rs: ReAct agent orchestration
- fluent-agent/mcp_client.rs: MCP protocol client
- fluent-agent/tools/string_replace_editor.rs: File editor tool

Documentation includes features, examples, and security notes.

Author: njfio