Every push to an open PR ran the full validate job, and pull_request
lacked the edited type, so removing WIP: would not start a run. pr-flow
therefore refused to open PRs early.
Skip validate on PRs titled WIP: or [WIP], add edited so un-WIP starts
the run, add workflow_dispatch for on-demand runs, and cancel superseded
PR runs (never push runs, since main's push uploads the package). The
workflow name and job id are unchanged, so the required check context
stays "Firefox Extension CI / validate (pull_request)".
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
