Part 3b: clarify the MHS reliability corpus

[AMBRA7592]

Summary

• add a dated, standalone prospective clarification for MHS reliability metric 4
• confirm the converted reliability corpus contains only eligible Conservative/Liberal cohort annotators
• exclude neutral, no-opinion, and null-ideology judgments from the reliability corpus, CONFIDENT-cell determination, and leave-one-out computation
• state explicitly that no threshold, metric, claim boundary, or Phase-2 authorization changes

Timing and scope

Recorded on 2026-07-19 before any real-MHS outcome run. Exactly one new documentation file; the original pre-registration, adapter, harness, instruments, tests, data, schemas, and README remain untouched.

Verification

• python3 -m unittest test_claims — 37 tests, 34 pass + 3 expected schema skips
• git diff --check clean
• no Phase-2 report, manifest, parquet, or outcome

Review gate

Do not begin Phase 2. Claude should verify this file records the owner's cohort-only decision faithfully, is dated prospectively, changes no protocol parameter, and is the only file in scope.