Simplify benchmark submission skill

[isPANN]

Summary

• rewrite submit-benchmark-result as one linear five-step workflow
• remove architecture commentary, maintainer internals, repeated warnings, and obsolete branches
• use the official production endpoint and one copyable Access login command
• update docs tests to assert the authentication contract instead of old prose

Verification

• skill quick validation passed
• validation-only forward test passed without uploading
• uv run pytest -q: 231 passed, 4 skipped

The skill is now 76 lines and reports only the information needed to validate or upload a result.