build: run CI on pushes to legacy-mfe

[arbrandes]

Description

ci.yml and lockfileversion-check.yml still push-trigger on master, which is now the App Repository, so pushes to legacy-mfe run nothing and only pull requests get CI. This points both push triggers at the branch that actually carries this tree. Part of #226.

LLM usage notice

Built with assistance from Claude.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.66%. Comparing base (edb1eba) to head (d2f8199).

Additional details and impacted files

@@             Coverage Diff             @@
##           legacy-mfe     #229   +/-   ##
===========================================
  Coverage       97.66%   97.66%           
===========================================
  Files              68       68           
  Lines            1671     1671           
  Branches          446      410   -36     
===========================================
  Hits             1632     1632           
  Misses             39       39           

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