[NSchatz]

Adds this repo's first CLAUDE.md: a 57-line card a session can read in seconds. It has four parts: what the package is, the gate, the hazard, and the rules that bind here.

The card

• What it is: a TypeScript library that validates and converts the date parts the @cosyte/* parsers return. It is tooling beside the parsers, not a parser.
• Gate: the verify ladder from .github/workflows/ci.yml, in its order, then pnpm check:no-internal-refs for the no-internal-refs / public-surface required check. It names all four required checks and points at .github/CI.md.
• Hazard: in our own words. A wrong conversion of a PHI-bearing date fails silently and nothing throws, which is why the package refuses rather than guesses.
• Rules: five, each tied to the failure behind it:
  • Read a fraction only through src/fraction.ts: fraction * 1e9 rendered 0.999999999999 s as .1 (S0188-cosyte-dates-companion-package-1: build @cosyte/dates to 0.1.0 #4, second commit, impl gate finding F1).
  • Decide skipped or repeated local times by comparing earlier and later: relabelling a thrown error told a fixed-offset zone it had a transition (S0188-cosyte-dates-companion-package-1: build @cosyte/dates to 0.1.0 #4).
  • Never hand-set version or a changelog heading: a hand-set 0.1.0 could never publish (Prepare the 0.1.0 release: version path, changeset, examples and metadata #7).
  • Keep .cosyte-release-tooling/** in the ESLint ignores: linting it failed a release run on main (Prepare the 0.1.0 release: version path, changeset, examples and metadata #7).
  • Never add pull_request: to scorecard.yml: the action refuses non-default refs (measured 2026-08-29, recorded in .github/CI.md).

The pnpm note comes from this session. corepack pnpm run attw failed here with ERR_PNPM_BAD_PM_VERSION: attw's nested pnpm pack resolved to a global pnpm 12, which refuses to switch versions under corepack. With a corepack shim first on PATH, every nested call gets the pinned 10.34.5 and the ladder passes.

The repo had no old CLAUDE.md, agent notes or CONTRIBUTING.md, so no text was moved and no links pointed at a dropped section. No changeset: nothing under src/ changes.

Every command the card names, and how it was checked (checked, not run)

named in the card	kind	check
pnpm install --frozen-lockfile	pnpm built-in	command -v pnpm -> /usr/local/bin/pnpm; CI installs pnpm through pnpm/action-setup
pnpm run build	script	jq -r '.scripts | keys[]' package.json lists build
pnpm run attw	script	lists attw
pnpm run examples	script	lists examples
pnpm run typecheck	script	lists typecheck
pnpm run lint	script	lists lint
pnpm run format:check	script	lists format:check
pnpm run test	script	lists test
pnpm run test:tz	script	lists test:tz
pnpm check:no-internal-refs	script	lists check:no-internal-refs
pnpm changeset	script	lists changeset
pnpm pack	pnpm built-in (named as what attw calls)	command -v pnpm
bash scripts/check-no-emdash.sh	binary + file	command -v bash -> /usr/bin/bash; git ls-files scripts/check-no-emdash.sh
corepack pnpm, corepack enable pnpm --install-directory <dir>	binary	command -v corepack -> /usr/local/bin/corepack
.github/workflows/ci.yml, .github/CI.md, .changeset/README.md, package.json, CHANGELOG.md, src/fraction.ts, eslint.config.js, .github/workflows/scorecard.yml	files	git ls-files <path>: each tracked
src/, test/, examples/, docs-content/	directories	git ls-files <dir>: each non-empty

.cosyte-release-tooling/** is an ignore pattern in eslint.config.js (present there), not a repo path. pull_request: is a workflow key.

Checks run

• wc -l < CLAUDE.md prints 57.
• The full verify ladder passed locally with pnpm 10.34.5 (Node 24): install, build, attw, examples, typecheck, lint, format:check (covers the root *.md), test (175 passed), test:tz (175 passed under each zone), em-dash scan. pnpm check:no-internal-refs passed too.
• rg -n '[\x{2013}\x{2014}]' CLAUDE.md finds nothing.

Harness-Task: 78

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