ci: add manual TrainerRank GPU validation

[bradhilton]

Summary

• request @bradhilton for changes under src/art/trainer_rank
• classify a narrow set of TrainerRank-critical Megatron changes
• keep 2x H200 validation waiting behind a GitHub environment approval
• run focused CP2, TP2, GDN, FlexAttention, dynamic-slot, and TrainerRank forward/backward parity coverage with a 20-minute GPU deadline

Validation

• repository quality hooks
• actionlint and shell/YAML parsing
• full non-critical classification path: GPU job skipped and final gate passed
• full critical path on the ART GPU image: environment approval, 2x H200 provisioning, 8 integration tests, all 16 TrainerRank request combinations under CP2 at depths 0 and 4, exact head-chunk/backward parity, final gate, and automatic teardown
• critical-path workflow completed in 9m38s: https://github.com/OpenPipe/ART/actions/runs/29160056611

The trainer-rank-gpu-validation environment is configured with @bradhilton as its required reviewer. No branch protection or required status check is added.