[lint] Fix pyrefly errors on main

[dlwh]

Repair the pyrefly failures currently blocking the main lint workflow. The fix restores the missing import in LM training, makes the labeled eval mixed-precision policy explicitly nullable, gives Jinja global mutation a type pyrefly accepts, and narrows completed trace eval metrics without relying on helper-predicate inference.

[claude]

Claude finished @dlwh's task in 1m 24s —— View job

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🤖 ### Code review

No issues found. Checked for bugs and CLAUDE.md / AGENTS.md compliance across all four modified files.

Reviewed changes

File	Change	Verdict
lib/levanter/src/levanter/callbacks/labeled_eval.py	mp: jmp.Policy → jmp.Policy | None (matches None default; call sites already None-guard)	✅
lib/levanter/src/levanter/main/train_lm.py	adds import os (resolves existing os.path.join usage at line 400)	✅
lib/levanter/src/levanter/tokenizers.py	wraps env.globals writes through cast(dict[str, Any], env.globals) — typing-only no-op	✅
lib/marin/src/marin/evaluation/trace_labeled_eval.py	inlines _is_completed_dataset_result as explicit isinstance(..., Mapping) guards (semantically equivalent — the helper was exactly those two checks)	✅

Method. 2 sonnet agents reviewed against the relevant CLAUDE.md / AGENTS.md files (root, lib/levanter/, lib/marin/). 2 opus agents independently audited the diff for bugs and broken call sites. One CLAUDE.md agent flagged the inlined isinstance check as "silent fallback replacing an assert" in trace_labeled_eval.py; on validation this is a false positive — the removed assert sat after _is_completed_dataset_result had already returned True, so it was a type-narrowing assertion that could never fire. Behavior is byte-equivalent.

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  · Branch codex/fix-main-pyrefly-lint

[rjpower]

i think these were fixed in a separate PR