Add no-sync-fs rule

[create-inc-service-account]

Summary

• Flags synchronous fs methods (readFileSync, writeFileSync, existsSync, etc.) and suggests fs.promises or fs/promises instead
• Detects both fs.xSync() member calls and destructured imports from 'fs'/'node:fs'
• Uses endsWith('Sync') instead of a hardcoded method list — future-proof against new Node.js fs additions
• Tagged as backend platform only
• Codifies the code style preference from our escher CLAUDE.md: "Always use fs.promises.* instead of sync variants"

Test plan

Scenario	Expected
npm test	All 350 tests pass (14 new for this rule)
fs.readFileSync(...)	Flagged with suggestion to use fs.promises.readFile
import { writeFileSync } from 'fs' then writeFileSync(...)	Flagged
import fs from 'node:fs' then fs.existsSync(...)	Flagged
await fs.promises.readFile(...)	Allowed
import { readFile } from 'fs/promises'	Allowed
someObj.readFileSync(...)	Allowed (not an fs import)

🤖 Generated with Claude Code