feat: added variables for MCP operation

[jwoehr]

Added 4 variables to the env.example that should be there.
In particular, one needs to set IBMI_ENABLE_EXECUTE_SQL=true to enable the ibmi-agent-skills

# Path to YAML tools (CLI --tools takes precedence)
# TOOLS_YAML_PATH=

# Enable built-in tools (default: false)
# IBMI_ENABLE_DEFAULT_TOOLS=false

# Enable execute_sql tool (default: false)
# IBMI_ENABLE_EXECUTE_SQL=false

# Restrict execute_sql to SELECT only (default: true)
# IBMI_EXECUTE_SQL_READONLY=true