[T6] `--precision` on `tether finetune` is a silent no-op

[DsThakurRawat]

Labels: bug, finetune, dx, priority:medium
File: src/tether/finetune/cli.py:70-72 (flag), src/tether/finetune/run.py:76 (validated), :129-131 (intentionally not passed through)

Description. --precision bf16|fp32 is accepted, validated, then omitted from the lerobot command ("intentionally NOT passed through"). It is honored for distill/SnapFlow, making the inconsistency worse.

Tasks.

• Either wire precision into the generated lerobot args/policy config, or reject --precision for the fine-tune path with a clear "not supported in v0.3" error.

Dedup. #45 standardizes hardware flags; not this. Not covered.