CC-518: Add immutable gate scope manifests

[screenleon]

Summary

• generate a content-addressed gate_scope_manifest_v1 before reviewer dispatch and bind it to the immutable gate subject
• record changed, renamed, and untracked paths, diff hunks, paired tests, sensitive signals, surface flags, and bounded peer/call-site/shared-helper expansion hints
• deliver the same manifest digest to sequential and parallel reviewers, with explicit truncation budgets and acceptance
• link and validate the manifest through gate_assurance_v3 for named consumers
• stream large expansion JSON through a file-backed jq input so complete manifests do not exceed Linux argv limits
• add schema, verifier, CLI/docs, and deterministic regression coverage

Why

Reviewers previously reconstructed scope independently from diff lists and prompts. That made renamed or untracked paths, paired tests, sensitive signals, and bounded adjacent context difficult to prove consistently across reviewers or synthesis. Large expansion sets also exposed an infrastructure failure when hundreds of JSON entries were passed to jq as one shell argument.

This change gives every selected reviewer the same immutable, digest-bound scope declaration. Truncation is fail-closed unless explicitly accepted, and accepted omissions remain visible with exact counts and reasons. The expansion implementation preserves the complete JSON input without relying on a single oversized argv entry.

Impact

Gate consumers can verify what scope was declared without treating that declaration as proof of review completeness. Historical v3 artifacts remain readable for generic inspection, while named policy consumers require valid linked scope evidence.

Validation

• PR gate: Final GO with critic and QA coverage; artifact validity, subject currentness, and policy applicability verified
• tests/bin/run-all-tests.sh: 97 passed, 0 failed, 0 skipped
• tests/bin/run-tests.sh --verify-full .pm-dispatch/test-results/latest-full.json: authoritative full result verified
• test-pr-gate: 198 passed, 0 failed, including tracked/untracked binary routing, complete manifest sharing, large expansion file input, and explicit truncation acceptance