docs: clarify primary type labels are issues-only (PRs use ownership labels) (#278)

[SamuelHassine]

Summary

Clarifies the shared label taxonomy so it can never be misread (by a human or an automated reviewer):

• Primary type labels (feature / bug / documentation) are applied to issues only, alongside the GitHub issue Type.
• Pull requests do not carry a primary type label — the type: title prefix conveys the type. PRs should instead carry an ownership label (filigran team / community) to differentiate the author, plus any useful area/workflow labels.

Updated .github/LABELS.md, .github/labels.yml, and the CONTRIBUTING conventions block.

Closes #278

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.34%. Comparing base (0a48f46) to head (1cfef09).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #279   +/-   ##
=======================================
  Coverage   70.34%   70.34%           
=======================================
  Files          49       49           
  Lines        1966     1966           
=======================================
  Hits         1383     1383           
  Misses        583      583           

Flag	Coverage Δ
connectors	70.34% <ø> (ø)

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