Fix two lint issues that cause `npm run lint` to exit non-zero:
1. README.md (MD028): Two consecutive blockquotes separated by a bare
blank line. Markdownlint treats this as one blockquote with an
illegal blank line inside. Replace the blank line with a `>`
continuation so both paragraphs stay in the same blockquote.
2. session-activity-tracker.js (eqeqeq): Three instances of `== null`
replaced with explicit `=== null || === undefined` guards to satisfy
the repo's `eqeqeq: warn` ESLint rule.
Closes#1366
