ci: run workflows on pull_request, restrict push to main

[MikaAK]

Summary

• All four workflows (Test, Coverage, Credo, Dialyzer) used on: push, which fires on every branch push and on PR sync events — every PR update kicked off two duplicate runs.
• Switched each workflow to:

  on:
    push:
      branches: [main]
    pull_request:

  which runs CI once per PR update plus once on main after merge (covering post-squash branch protection / status checks).

Test plan

• Open this PR — CI should kick off via the pull_request trigger and report Test / Coverage / Credo / Dialyzer once each.
• After merge to main, those four checks fire again on main via the push: branches: [main] trigger.
• Pushing to a feature branch without an open PR should NOT trigger any workflow runs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.46%. Comparing base (ff1c3df) to head (a25c660).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
- Coverage   83.46%   83.46%   -0.01%     
==========================================
  Files          22       22              
  Lines         617      635      +18     
==========================================
+ Hits          515      530      +15     
- Misses        102      105       +3     

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