Add two defenses against malicious plugins from uncontrolled sources:
1. **Pinned-ref enforcement** (resolveAndStage): github:// install/download
specs without a #<tag/sha> suffix are now rejected with HTTP 422. A
mutable default-branch tip could change between audit and install,
silently swapping in untrusted code. Override via PLUGIN_ALLOW_UNPINNED=true.
2. **SHA-256 content integrity** (installRequest.sha256): callers may
supply the expected hex SHA-256 of the fetched plugin.yaml. When present,
resolveAndStage verifies the digest after staging; a mismatch aborts the
install with HTTP 422 and cleans up the staging dir.
Updated TestPluginDownload_GithubSchemeStreamsTarball to use a pinned ref
(#v1.0.0) so it reflects the new security requirement.
Tests: 4 new (TestPluginInstall_SHA256Mismatch_AbortsInstall,
TestPluginInstall_SHA256Match_Succeeds, TestPluginInstall_UnpinnedRef_Rejected,
TestPluginInstall_PinnedRef_Accepted). All 15 packages green.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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