docs(readme): Fix broken Copilot badge and rename Merged PRs badge

[Copilot]

Two badge issues flagged in review on the README status badge additions.

Changes

• Removed the Copilot code review workflow badge — referenced a non-existent workflow path (.github/workflows/copilot-pull-request-reviewer/copilot-pull-request-reviewer), producing a permanently broken badge
• Renamed "Merged PRs" → "Closed PRs" and corrected the URL filter from is%3Amerged to is%3Aclosed — shields.io's github/issues-pr-closed endpoint returns all closed PRs (merged + unmerged); the prior label was misleading

-[![Copilot code review](https://github.com/mpaulosky/IssueManager/actions/workflows/copilot-pull-request-reviewer/copilot-pull-request-reviewer/badge.svg)](...)
-[![Merged PRs](...?q=is%3Amerged+is%3Apr)]
+[![Closed PRs](https://img.shields.io/github/issues-pr-closed/mpaulosky/IssueManager?color=6f42c1)](https://github.com/mpaulosky/IssueManager/pulls?q=is%3Aclosed+is%3Apr)

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[Copilot]

Copilot wasn't able to review any files in this pull request.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.22%. Comparing base (26c6da1) to head (1802ac2).

Additional details and impacted files

@@                   Coverage Diff                   @@
##           docs/readme-badges      #21       +/-   ##
=======================================================
+ Coverage               34.02%   46.22%   +12.20%     
=======================================================
  Files                      20       25        +5     
  Lines                     288      411      +123     
  Branches                    7       17       +10     
=======================================================
+ Hits                       98      190       +92     
- Misses                    189      216       +27     
- Partials                    1        5        +4     

see 5 files with indirect coverage changes

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