v2.0.1

Fix Chrome MV3 warning: per-browser background key

Chrome warned "'background.scripts' requires manifest version of 2 or
lower" because the manifest carried both service_worker (Chrome) and
scripts (Firefox). Firefox has no service_worker background (bug
1573659), so one shared manifest always warns on one browser.

Committed manifest.json is now Chrome-canonical (service_worker only) —
no Chrome warning on unpacked load. The release builds rewrite background
to the scripts form for the Firefox zip/.xpi via a jq step, so Firefox
loads warning-free too. Same sw.js everywhere.

- manifest.json: drop background.scripts
- pack.yml: emit overhead-chrome-v*.zip + overhead-firefox-v*.zip
- sign-firefox.yml: jq-inject scripts into the staged manifest
- README: two-zip install, updated "How it works"
- bump 2.0.0 -> 2.0.1

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>