[farhan-syah]

Summary

Stops CI from repeating work it has already done.

• Branch-push trigger removed (ci.yml): each PR commit used to run CI twice, once for push and once for pull_request. Branch pushes without a PR now run nothing. "Run workflow" still runs on demand.
• Content-keyed results:
  • CI uploads a marker artifact ci-pass-<tree> on success, where <tree> is the tree of the PR's merge commit.
  • Screenshots uploads screenshots-<key>. The key hashes only the files that decide the pixels.
• Merges reuse PR results: on a push to main, a short gate job looks up the key first.
  • Merge leaves the tree or key unchanged: CI skips the checks, and Screenshots publishes the PR's artifact with no new capture.
  • main moved under the PR: both run in full.
• Path filters:
  • CI ignores docs, licence files, label config, and the screenshot workflow and scripts.
  • Screenshots runs only when nodedb-studio/, Cargo.toml, Cargo.lock, its scripts, or its workflow change.
• Concurrency: a newer push to the same PR, or to main, cancels the run it supersedes.

Notes

• YAML anchors: path lists are shared between triggers. This PR's own run is the first check that GitHub accepts the files.
• Marker retention: markers expire after 30 days. A PR merged after that runs CI once on main.
• First merge recaptures: feat(screenshots): capture every screen to PNG in CI #9's artifact used the old name screenshots, so the first merge after this one recaptures once.

Test plan

• On this PR, CI runs gate then check, and uploads ci-pass-<tree>.
• On this PR, Screenshots runs key, capture, and report.
• After merge, CI on main shows check skipped.
• After merge, Screenshots on main skips capture and publish reuses the PR's artifact.