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Summary

Adds regression tests that enforce the plugin contribution policy in
CONTRIBUTING.md: signed, allowlisted catalog installs, no implicit capability
grants, no plugin code execution by the management API/CLI, and no return of the
obsolete blanket "local only" rule.

Problem

The policy text was updated in #532, but nothing kept it from regressing. #427's
acceptance criterion explicitly asks for an accurate, testable policy, and
the old local-only rule could be reintroduced by a routine docs edit.

Solution

• Added tests/test_contributing_plugin_policy.py.
• It normalizes whitespace, then asserts:
  • no local only, local-only, or local/trusted wording remains;
  • the signed/allowlisted catalog model and its config variables are present;
  • Ed25519 signatures, integrity hashes, and the client-URL prohibition remain;
  • no-implicit-grants and no-code-execution rules remain.
• No existing policy text was changed, so this does not duplicate docs(plugins): document the marketplace security model #532.

Validation

• python -m pytest tests/test_contributing_plugin_policy.py -q — PASS (3 passed)
• python -m ruff check tests/test_contributing_plugin_policy.py — PASS
• python -m black --check tests/test_contributing_plugin_policy.py — PASS
• Repo-wide ruff/black remain blocked by the pre-existing malformed
  app/services/invoice_export.py and tests/test_invoice_export.py files
  merged in feat(billing): export invoices as CSV and PDF with size bounds (#483) #535.

Risk

Test-only change. It codifies the current documented policy and will fail if the
stale local-only rule or the safety rules are removed.

Issue

Closes #427