fix(tests): add structural-regression-guard annotation for shebang assertion

The shebang check added in #3105 falls under structural-regression-guard:
the portability constraint (#!/usr/bin/env bash vs #!/bin/bash) cannot
be caught at runtime on distros that have /bin/bash. Adding the annotation
satisfies the adversarial review finding that new tests cannot use
pending-migration-to-typed-ir as their exemption category.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/bug-2136-sh-hook-version.test.cjs

@@ -3,6 +3,13 @@
 // "Prohibited: Raw Text Matching on Test Outputs". Per-file review may
 // reclassify some entries as source-text-is-the-product during migration.
 
+// allow-test-rule: structural-regression-guard
+// The shebang line must be `#!/usr/bin/env bash` (PATH-resolved) rather than
+// `#!/bin/bash` for cross-distro portability (NixOS, minimal Alpine do not
+// ship /bin/bash). This is an architectural constraint that cannot be verified
+// by executing the hooks — they run fine with either shebang on distros that
+// have /bin/bash, so only a source assertion catches a future regression.
+
 /**
  * Regression tests for bug #2136 / #2206
  *