docs: add comprehensive MCP documentation and examples
- docs/mcp.md: Full MCP documentation covering prerequisites, configuration,
transports (stdio + HTTP), security (env filtering, credential stripping),
reconnection, troubleshooting, popular servers, and advanced usage
- README.md: Add MCP section with quick config example and install instructions
- cli-config.yaml.example: Add commented mcp_servers section with examples
for stdio, HTTP, and authenticated server configs
- docs/tools.md: Add MCP to Tool Categories table and MCP Tools section
- skills/mcp/native-mcp/SKILL.md: Create native MCP client skill with
full configuration reference, transport types, security, troubleshooting
- skills/mcp/DESCRIPTION.md: Update category description to cover both
native MCP client and mcporter bridge approaches
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