Support X auth token posting with emusks

[bupd]

Summary

• add emusks-backed Twitter/X posting when TWITTER_AUTH_TOKEN or AUTH_TOKEN is present
• keep existing official Twitter API credentials as a fallback
• upload media through emusks with alt text and support emusks response URLs
• document auth-token setup and optional emusks client/endpoint/proxy envs

Verification

• bun run build
• bunx eslint src/strategies/twitter.js src/bin.js tests/strategies/twitter.test.js
• bunx mocha tests/strategies/twitter.test.js --exit --forbid-only
• bun tests/pkg.test.mjs
• bun run test:attw
• bun run test:unit
• live smoke: posted to X with AUTH_TOKEN, then deleted the test post