ci: add publiccode.yml validation workflow

[publiccode-pr-bot]

This PR adds a GitHub Actions workflow that validates publiccode.yml on every pull request using italia/publiccode-parser-action@v1.

No behavior change to the code, just CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.23%. Comparing base (1626e92) to head (38c9a6d).
⚠️ Report is 107 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11986      +/-   ##
==========================================
- Coverage   89.60%   86.23%   -3.37%     
==========================================
  Files        1306     1306              
  Lines       28888    28888              
  Branches     5979     5979              
==========================================
- Hits        25884    24912     -972     
- Misses       3004     3976     +972     

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