fix: collect all PR-URL entries in getAltPrUrl()

[watilde]

getAltPrUrl() was only ever returning the first PR-URL entry in a commit message body. The root cause was PR_RE missing the global flag (g), so text.match(PR_RE) returned a single match-result array (full match + capture group) instead of an array of all matching lines.

This follows the existing pattern in the file where FIXES_RE/FIX_RE and REFS_RE/REF_RE are paired as global/non-global variants. Added PRS_RE as the global counterpart and updated getAltPrUrl() to use it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.13%. Comparing base (4ea22b8) to head (6a85c35).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1090      +/-   ##
==========================================
+ Coverage   64.93%   65.13%   +0.20%     
==========================================
  Files          45       45              
  Lines        6938     6939       +1     
==========================================
+ Hits         4505     4520      +15     
+ Misses       2433     2419      -14     

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