[Feat] support action forward dynamics finetuning - #283

Update Summary

Addressed the review comments in the latest commit:

• Fixed cookbook-relative paths and anchored them to the launcher location, so the script works regardless of the caller's working directory.
• Removed the dependency on the missing _sft_launcher_common.sh; the launcher is now self-contained with input validation, logging, overrides, and single-/multi-node torchrun setup.
• Added action_fd_droid_posttrain.md with prerequisites, dataset layout, setup instructions, config validation, training commands, and output details.
• Added framework-root resolution and model-config validation so framework-relative assets resolve correctly.