fix: update minimal example README key/mode and add unit test (fixes #218)

[aipd506]

Summary of Changes

Resolves #218:

• Updated examples/minimal/README.md to reference the real pinned trusted_root_issuers key from ca2a-config.yaml (eda38c44...eb162).
• Corrected description from 'advisory' to 'enforcing' to match ca2a-config.yaml.
• Added tests/unit/test_example_minimal.py executing ca2a verify-chain and ca2a validate-config via cli_main, asserting verified: true and ok: provider=software-only enforcement=enforcing.

Acceptance Criteria Verification

• README uses the real key and says 'enforcing'.
• New unit test test_example_minimal.py runs both commands through ca2a_runtime.cli.main and asserts output.

[github-actions]

Thanks for this, @aipd506. Closing it for now, and it is not about the change itself.

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