ci: publish regtest apk artifact and link it on prs

[ovitrif]

Every regtest build that someone wants to test on a device (see the Slack thread about parsing the new Pubky auth claim link) currently requires a manual Release Internal dispatch or a local build. CI already builds the regtest app with the real Firebase google-services.json on every PR push, but throws the APK away.

Requirements

• CI publishes the regtest debug APK (assembleDevDebug, universal APK) as a workflow artifact on every non-draft PR push and on every push to master.
• The build uses the GOOGLE_SERVICES_JSON_BASE64 secret so Firebase push notifications work in the artifact, same as the E2E builds.
• The artifact is named bitkit-dev-debug-apk_<run_number> and kept for 30 days.
• The workflow posts one sticky comment on the PR with the artifact link, the built commit SHA, and the run link; later pushes update that comment instead of adding new ones.
• No extra build job: the existing CI workflow build step is reused, so the cost per push stays the same.

Acceptance

• A PR push produces the artifact on the CI run page and a single comment on the PR linking to it.
• A second push to the same PR updates the comment with the new artifact link and SHA.
• A merge to master produces the artifact on the CI run for that commit.
• The APK installs on a regtest device and receives Firebase notifications.