Add dev-feature preservation gate and change schedule

[Phlip79]

Two changes to the main-to-dev nightly sync workflow:

1. New workflow .github/workflows/nightly-sync-dev-preservation-gate.yml that runs deterministically on every push to a main2dev/* PR (no LLM in the loop). For each non-exempt file the sync touched, it computes:

   (lines on origin/dev) - (lines on origin/main) - (lines in tree)
   

   and posts a sticky PR comment listing every line that satisfies all three. The workflow fails if any non-exempt file has a non-empty result, blocking the PR from being marked ready.

   Catches the most common sync regression: feature lands on dev at T0, lands on main at T1>T0, sync runs in between, -X theirs drops dev's feature wherever main happened to touch nearby lines. Recent examples this would have caught:

   • _forward_mlp_router(input_ids=None) in transformer_layer.py
   • num_sms_preprocessing_api=... kwarg in token_dispatcher.py
   • self._maybe_record_overload_factor(...) call in moe_layer.py
   • parse_and_validate_args import in gpt_dynamic_inference_with_coordinator.py
   • args.dynamic_context_parallel references in data_samplers.py / utils.py / training.py
   • "Packing Scheduler" section in datasets/readme.md

   Files in the skill's "Files to Override from Main" list
   (training.py, utils.py, data_samplers.py, initialize.py,
   layer_wise_optimizer.py) report as warning rather than error,
   matching the skill's intent that main may legitimately win there.
   pyproject.toml / uv.lock / docker/Dockerfile.ci.dev and CODEOWNERS
   are skipped entirely (always dev's by skill rule).

   The job also publishes a prompt-addendum (on workflow_dispatch only)
   that can be pasted into the sync-bot prompt so the agent fixes
   violations proactively and the deterministic gate stays green.

2. Schedule change in .github/workflows/nightly-sync-main-to-dev.yml: from daily at 21:00 UTC to twice-weekly (Monday + Thursday) at 15:00 UTC, which is 8 AM PDT (7 AM PST in winter, since GitHub Actions cron is UTC-only and does not follow DST).

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[Phlip79]

/ok to test ac2e39b