feat(config): centralized SDK settings, server config, config CLI, docs/tests

[AKKI0511]

• Introduces centralized configuration in agentconnect.config with a single global settings object and YAML-based overrides.
• Adds configuration CLI: agentconnect config {init, show, validate}.
• Enhances agentconnect.servers with env-driven server settings and unified vector store options; easier programmatic startup.
• Adopts centralized settings across clients, agents, registry, communication hub, MCP, and utils.
• Reorganizes env examples: root example.env (SDK) and demos/.env.example (demo-local).
• Updates Google provider/types for Gemini model changes.
• Removes legacy core configuration and temporary payment constants.
• Updates docs and expands tests for servers, config, and MCP.

Breaking changes

• Legacy core configuration module and payment constants removed in favor of centralized settings. Minimal migration may be required for custom integrations.

Base: develop
Compare: feature/config-overhaul