feat(scanner): reject plugin checksum files without a sibling signature

[seonghobae]

Successor of #1174 / issue #1099. Does not Close those. Checksum mismatch stays #1169. Hosted deploy stays #1174. Pass is not Noema admission.

Unique delta

Fail closed when a first-party checksum file lists digest rows but has no sibling signature artifact (claude-plugin-unsigned-checksum, CWE-347):

• matching SHA256SUMS / *.sha256 without .sig, .asc, .gpg, .bundle, or cosign.bundle fails admission
• a non-empty sibling signature file is not this class
• a zero-byte .sig or a symlink .sig is not a signature
• missing checksum files and comment-only checksum files stay not-this-class
• digest mismatch stays claude-plugin-checksum-mismatch and can fire with this class
• Cosign/GPG network verification is not performed; snippets are checksum filenames

#1169 remains owner of digest disagreement.

Evidence

• RED 5a7abb3 then GREEN eb2389e

• tests/test_claude_plugin_unsigned_checksum.py plus sibling mismatch lock

• predecessor checksum plugin coverage suite 416 passed on Python 3.13

• no hosted workflow is created on this stacked custom base; keep Draft until stacked integration evidence is available

• Normal two-parent descendant aa5128fb115668ce8abf869dd1d711e72fe09baa integrates feat(scanner): fail closed on plugin terraform apply and helm install #1173 command-context RED/GREEN without force-push; compare to current parent d8f5f76400f9... is 4 ahead / 0 behind and preserves this PR's unique delta.

• Candidate source AST passes; the final stacked detector preserves 3/3 negative command-context and 2/2 later-real-command probes. Custom-base hosted workflow evidence remains absent, so Draft stays required.

Relates to #1099. Relates to #1174.

Current quoted-command context repair

This descendant inherits #1173 RED d7384e53d44a426356a73e24b2b9cc3fe153de27 and final canonical repair 3a5b14771cc7abb6553fa1ec9bcd8b62acc947ac without copying a leaf workaround. Exact head: 77f7ffa03cc97670ae66e329b6b811e6264867a4; current parent: 2967d093985f96dd0830cd600b86f7f07c7235df; Git compare is 0 behind and preserves this PR's unique detector/test delta. The top #1183 source AST and extracted helper probes pass quoted/reporting negatives while preserving direct and command-substitution positives. This custom-base PR stays Draft until its own hosted integration and independent review are terminal.

Current assignment-value context repair — 2026-09-08

This normal two-parent descendant inherits canonical #1173 RED 90ae232e852b3d01463d978f9bccd9157c341fe7 and GREEN 1e6a3eb43a5bc2d146656af13e7fa6cad2a77315 without a leaf workaround. Exact head: 29011cca6f465dbd988465c6ff67fe7caa18223b; current parent: 8b1965da8972a627d40fb7b1866b6828007a4a13; Git compare is 0 behind and preserves this PR's unique detector/test delta. Assignment-value negatives and environment-prefix/direct/substitution positives are present in the exact tree. This custom-base PR remains Draft until its own hosted integration and independent current-head review are terminal. Earlier head strings in this body are historical evidence only.

Current heredoc-safe restack — 2026-09-08

Canonical #1173 exact head 184b4b4f55906284ac1d8059e46cec8f0d9cfc9a fixes closed literal here-document payload false positives with RED 6f6be77d... → GREEN 12faf2f.... This branch was integrated non-destructively on parent c069e93ed7ca0b82fd19179a6b68f671c3d0be20; exact head is bb9cd1dd6af007f2b1e85978974150480da3fad6. GitHub compare reports behind_by=0, ahead_by=13; the PR's unique detector delta remains in the tree. Exact-head hosted integration and independent review are not terminal, so this PR remains Draft. Earlier head strings are historical evidence only.

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