feat(action): compute baseline coverage delta for PRs

[felmonon]

Summary

The Action currently publishes the current coverage report but does not compute a baseline delta (noted in the README). For a coverage tool, the most actionable CI signal is how coverage changed on a PR.

Proposed behavior

• Read a baseline report (from the target branch, a committed file, or a previous run artifact)
• In the sticky PR comment, show +/- deltas for coverage %, mocked calls, unmocked calls, stale handlers, and unsupported count vs baseline
• Optionally fail the job when coverage drops beyond a threshold

Acceptance criteria

• Action can read a baseline report and compare against the current run
• Sticky comment shows deltas for each summary metric
• Missing baseline degrades gracefully to current-only output
• Documented in README with an example workflow

Why

PR-over-PR change ("coverage dropped 4% on this PR") is far more actionable than a bare current snapshot. Likely the single highest-impact feature for the Action.