Add composable config system with includes support

[yichao-liang]

Summary

• Adds a composable YAML config system with includes support for experiment configurations
• Introduces modular config files for environments, approaches (oracle, agents), and common settings under scripts/configs/predicatorv3/
• Extends cluster_utils.py with config loading and merging logic that resolves includes directives to compose final experiment configs from reusable fragments

Test plan

• Verify config loading with includes resolves correctly
• Test that existing predicator_v3.yaml configs still work
• Run experiment submission with composed configs