feat: add cookie health check and safe cookie rotation workflow

[lumishoang]

Summary

This PR adds a safer, operator-friendly cookie rotation flow for Twitter auth stability.

Added

• npm run cookie:check (src/cookie-check.mjs)
  • Validates cookie file structure and performs a lightweight auth health-check.
• npm run cookie:replace -- <path> (src/replace-cookies.mjs)
  • Validates incoming cookie file (auth_token, ct0)
  • Backs up current data/cookies.json to data/backups/
  • Health-checks new cookie file before atomic swap
  • Leaves current cookie untouched on failure
• src/lib/cookie-health.mjs
  • Shared validation + health-check + error classification logic

Runtime hardening

• Startup preflight in src/index.mjs now checks cookie health first.
• Bot exits early with clear Telegram alert when cookie/auth is invalid.

Docs

• Added guides/05-cookie-rotation.md
• README now includes cookie check/replace commands

Why

Current flow detects session-expiry after failures. This PR adds proactive and safe rotation with lower operational risk.

Backward compatibility

• Existing npm start and setup flow remain intact.
• New commands are additive.