chore: clean dead todo.org doc-links and file three docs to outbox

I removed three dead `file:` links in todo.org. They pointed at docs that were never written: testing-strategy.org, firmware-cleanup.org, and PLAN-browser-themes.org. Each task body already carries that content inline, so the links were dead weight. I also reworded the Testinfra task's lead-in that claimed the testing-strategy doc exists.

I filed three resolved reference docs into assets/outbox/: the calendar-sync scrub note, the tmux copy-mode handoff, and the 2026-05-19 lint follow-up report.

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