chore(claude): learn from #278

[claude]

What this does

Captures one rule from the reviewer feedback on #278 and adds it to
CLAUDE.md as hard rule #6:

Tests that mutate module-level state must save and restore it via
try/finally.

Context: #278 added a second module-level dedup flag
(_SKIP_TIMESTAMP_WARNED) alongside the existing
_CONTROL_MODE_WARNED in src/opentau/datasets/lerobot_dataset.py.
The first round of that PR's test mutated the flag without restoring
it; the reviewer's @claude fix the nit comment asked for a
try/finally save/restore so the test doesn't leave the flag
flipped for subsequent tests in the same pytest-xdist worker. The
shape of the fix landed as test_skip_timestamp_warning_emitted_once_per_process
in tests/datasets/test_datasets.py, which the new rule points
to as the canonical example.

This is a recurring concern — there are now two such flags in the
same file, and the dedup pattern is likely to grow as more
mixture-wide warnings get added. Recording it here means future
tests in this codebase get the convention right on the first try
without re-litigating the nit.

No code changes; documentation only.

How it was tested

• N/A — single-line addition to CLAUDE.md, no executable code touched.

How to checkout & try? (for the reviewer)

git fetch origin claude/learn-from-278
git checkout claude/learn-from-278
git diff main -- CLAUDE.md

Checklist

• I have added Google-style docstrings to important functions and ensured function parameters are typed.
• My PR includes policy-related changes.
  • If the above is checked: I have run the GPU pytests (pytest -m "gpu") and regression tests.

🤖 Generated with Claude Code

[claude]

[claude-review] summary for commit 65f6b63

No blocking issues found.

• nit — CLAUDE.md:35 — Companion test test_control_mode_warning_emitted_once_per_repo (tests/datasets/test_datasets.py:1010) calls _CONTROL_MODE_WARNED.discard(test_repo) before the assertion but has no try/finally to restore prior membership; under the new rule it should follow the same save/restore shape. Out of scope for this docs-only PR, but a natural follow-up since the rule explicitly cites _CONTROL_MODE_WARNED as one of the flags it covers.

[shuheng-liu]

Not worth an entry in project-level CLAUDE.md