(MOT-4381) refactor(harness): finish objective E2E assessment migrations

[ytallo]

Summary

• add an unavailable assessment result for checks skipped behind an explicit prerequisite gate
• migrate receiving_operation, reactive_automation, and shell_coder_sandbox to canonical assessments
• preserve prerequisite and supplemental gates without duplicating failures
• remove the superseded manual award helper after all objective scenarios migrate

Validation

• cargo fmt --manifest-path harness/Cargo.toml --all -- --check
• cargo test --locked --manifest-path harness/Cargo.toml -p harness-e2e
• cargo clippy --locked --manifest-path harness/Cargo.toml -p harness-e2e --all-targets -- -D warnings
• git diff --check

Depends on #757

Fixes MOT-4381

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skill-check — worker

0 verified, 56 skipped (no docs/).

Layer	Result
structure	✓
vale	✓
ai	✓
render	✓

Four for four. Nicely done.