docs(claude): always existsSync for file existence (#594)

Add shared-standards rule: use existsSync from node:fs. Forbid
fs.access, fs.stat-for-existence, and async fileExists wrappers.
Canonical import form: `import { existsSync, promises as fs } from 'node:fs'`.

Author: jdalton

CLAUDE.md

@@ -22,6 +22,7 @@
 - Backward Compatibility: 🚨 FORBIDDEN to maintain - actively remove when encountered (see canonical CLAUDE.md)
 - 🚨 **NEVER use `npx`, `pnpm dlx`, or `yarn dlx`** — use `pnpm exec <package>` for devDep binaries, or `pnpm run <script>` for package.json scripts. If a tool is needed, add it as a pinned devDependency first.
 - **minimumReleaseAge**: NEVER add packages to `minimumReleaseAgeExclude` in CI. Locally, ASK before adding — the age threshold is a security control.
+- File existence: ALWAYS `existsSync` from `node:fs`. NEVER `fs.access`, `fs.stat`-for-existence, or an async `fileExists` wrapper. Import form: `import { existsSync, promises as fs } from 'node:fs'`.
 
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