Remove submission schema version field

[isPANN]

What changed

• remove schema_version from runner output, intake validation, private scored results, and both JSON schemas
• have the official scorer recognize the current ledger-backed structure directly
• retain shape-based handling only for historical local data; official submissions require the current ledger fields
• document the current problem-reductions tag/full commit and pred version in README
• clarify that the runner records library_commit and the scorer owns the round gate
• add drift tests linking README to the code-level round pins

Validation

• uv run pytest -q: 217 passed, 4 skipped
• focused runner/submit/ledger/scorer/docs tests: 79 passed
• generated a fake runner submission, confirmed schema_version is absent, and passed benchmark.submit --dry-run --test
• simplify review: reuse, quality, and efficiency passes completed; actionable cleanup applied