docs: correct the same two claims where the README audit didn't reach

[rdtiv]

#5 audited README.md on the premise that it was the one document carrying claims no gate covers. A review of that PR found the same two claims still standing in three files it did not touch — so the corrected version was losing 3-to-1 across the repo. This is the other three.

What changed, and what it was checked against

Claim	Reality in the code
"the --env-file flag every script in this project uses"	docs/setup-mac.md:92. package.json has 16 scripts; usage, typecheck, typecheck:py and verify:docs pass no --env-file. This document is read before the README, so it was the first place a beginner met the overclaim. Now uses README's corrected wording verbatim.
running the lessons from inside pyweather/ "would quietly create a SECOND usage.csv in there"	pyproject.toml:4-8. pyweather/usage.py:67 is LEDGER = Path(__file__).resolve().parent.parent / "usage.csv" — it cannot, regardless of cwd. #5's own body verified this. Rewritten to explain why that line exists rather than describe a hazard the code already prevents.
"runs the same lessons through uv, from the repo root, not from inside pyweather/"	CLAUDE.md:64. Contradicted both the README as rewritten by #5 and CLAUDE.md:193, which already stated the correct rationale. Since CLAUDE.md is the file every agent loads, the stale half would have won by default.
verify:docs is "the four gates — compile, ordering, diff, coverage"	CLAUDE.md:203. It runs six: structure and command parity repo-wide, then those four per document. Nothing warned that adding an npm script without a matching [project.scripts] entry fails the parity gate. The three NPM_ONLY exemptions are now named.

The blind spot this makes explicit

CLAUDE.md described the comment stripper as a mechanism without saying what the mechanism costs. Because comments are stripped from both sides before the diff gate compares them, a teaching comment can contradict its own listing indefinitely while all six gates stay green.

That is not hypothetical — it is exactly how the Part 7 timeout claim survived a passing CI in six places, corrected in #5. In this repo the comments are the teaching, so the hole is worth stating where agents will read it before they edit.

Gates

Run in a clean worktree off origin/main, exit codes checked individually:

• npm run typecheck → 0
• npm run typecheck:py → 0
• npm run verify:docs → 0

All keyless. No API call was made, billed or otherwise.

Not included

docs/setup-windows.md was checked and needs nothing — its Node 20.6 note mentions --env-file only in a parenthetical and makes no claim about which scripts use it.

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