Make browser extension checkout and CI reproducible

[jt55401]

Summary

• pin canonicalization to the immutable v0.2.2 release
• build the browser-client sibling at a fixed merged commit in CI
• run lint, tests, type checking, and each browser build through documented npm scripts
• replace the obsolete ESLint setup and remove the unused package token
• document the required checkout layout and current commands

Verification

Fresh Node 22 container with isolated dependencies:

• browser-client npm ci --ignore-scripts and npm run build
• browser-reference npm ci --ignore-scripts
• npm run lint (0 errors)
• npm test (54 tests)
• npm run typecheck
• npm run build:chromium
• npm run build:firefox
• npm run build:safari
• actionlint .github/workflows/ci.yml