Treat safe context prohibitions without weakening fail-closed checks

[yui-stingray]

Summary

• suppress built-in context findings only for bounded direct, shared, and modal prohibitions, including common Markdown wrappers
• keep conditional, mixed, and double-negation forms fail-closed, including chained punctuation and conditional provided forms
• cache identical lines within each scanned file to preserve the existing 1 MiB input and isolated execution budgets without changing custom rule semantics
• keep the report schema, CLI contract, and public evidence format unchanged

Validation

• full pytest -q
• focused context, inventory, contract, redaction, and consumer tests
• repeated maximum-size single-line and ten-rule corpus checks under the existing execution budget
• Agent-Guard Bench: 60 cases, overall F1 0.973451, context FP/FN 0
• self-dogfood context check: 0 findings
• python scripts/check_changelog.py
• git diff --check

Dependency

This PR is stacked on #47 because it relies on the bounded isolated context scan. Retarget it to master after #47 merges. It does not merge or publish the frozen release PR #43.

Related repositories

agent-policy has no package or evidence-contract dependency on this matcher change. The toolkit remains pinned to the published yui-agent-guard==0.3.4; its exact hash pin and committed evidence should be refreshed only after a compatible package release.

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[chatgpt-codex-connector]

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