fix: add missing ENERGY_SAVING_MODE variable to oar.conf

[faureadr]

Context

The ENERGY_SAVING_MODE variable is functional within the OAR3 source code, but it was completely missing from the default oar.conf template file. This made it difficult for administrators to discover and configure the coordination mode for automated power management.

Changes

This PR adds the #ENERGY_SAVING_MODE="metascheduler_decision_making" variable to the ENERGY SAVING section of the default oar.conf file.

• It is commented out by default to match the file's standard layout.
• It includes a brief comment explaining that this option coordinates power management decisions and is required for OAR to automatically shut down and wake up nodes.