feat(pre-push-safety-net): cover combined push failure paths

.kiro/specs/pre-push-safety-net/implementation-group-8.md

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+# Group 8 implementation and qualification
+
+The maintainer authorized group 8 on 2026-09-13. Fresh agent dispatch failed with
+`agent thread limit reached`; the explicit manual Kiro fallback applies. Reviews
+are main-context reviews, not independent reviewer evidence. Joint rollout gates
+remain unchanged; implementation progress does not imply capability eligibility.
+
+## Task 8.1 brief
+
+Requirements 1.1–1.4, 2.2, 2.3, 4.2, 4.4, 5.2, 5.5, 7.1, 7.4 and 8.2;
+design: Snapshot Adapter, Cache/Engine Boundary, Push runner and Result/Hook Adapter.
+Combine immutable non-HEAD/tag/force/new-branch/deletion updates, unusual paths,
+external boundaries, physical vendor exclusion, census failure and interrupted report
+publication. Use production transaction/backend seams; preserve source/index/refs,
+truthful scope and prior artifacts, and prove worker cleanup. Tests only: no production
+behavior or feature toggle introduced. Existing focused suites retain the full cache,
+missing-base, hook-exit and ordinary-scan regressions.
+
+## Status Report
+- STATUS: READY_FOR_REVIEW
+- TASK: 8.1
+- RED_PHASE_OUTPUT: N/A: tests-only integration coverage; no production behavior changed.
+- TESTS_RUN: Three added combined controls pass. Full suite 1299 passed, 9 skipped in 117.83s; ruff, formatting and mypy pass.
+
+## Review Verdict
+- VERDICT: APPROVED
+- TASK: 8.1
+- MECHANICAL_RESULTS: actual new tests inspected, full regression suite passes, no placeholders or secrets; test-only integration boundary maintained.
+- FINDINGS: mixed push control verifies all three exact comparisons and four dispositions, readable snapshot bytes, live state preservation and external-content exclusion. Failed census control inspects physically filtered source before returning missing answers. Interrupted writer control checks previous bytes and absence of its process PID.
+- SUMMARY: Combined failures remain explicit and do not masquerade as clean checks.
+
+## Verification Result
+- STATUS: VERIFIED
+- CLAIM_TYPE: TASK
+- CLAIM: Task 8.1 integration regression coverage delivered.
+- EVIDENCE: `/tmp/group8-1-focused.log`: 3 passed; `/tmp/group8-1-full.log`: 1299 passed, 9 skipped; static checks exit 0.
+- GAPS: These controlled failures do not qualify real detector capabilities or representative latency.