fix(av-surface): no runnable base64-to-shell line on disk

PLAN § v8.1.3 tillegg a. The bash-evasion gallery's T8 comment held a
complete base64-decode-into-shell line whose 16-char blob decodes to a
home-directory deletion, under probe (b)'s 24-char floor. Probe (b2)
found two more of the same shape, fixed here too: the gallery README's
T8 row and tests/e2e/attack-chain.test.mjs (an /etc deletion). Chosen
over an exception list because the gate should have none.

The gallery and the e2e test now encode the blob at runtime from the
same fragments; the README shows a placeholder. Gallery: 10/10 blocked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/bash-evasion-gallery/README.md

@@ -35,7 +35,7 @@ techniques below are caught.
 | T5 | IFS word-splitting | `rm${IFS}-rf${IFS}$HOME` | `rm -rf $HOME` |
 | T6 | ANSI-C hex quoting | `$'\x72\x6d' -rf $HOME` | `rm -rf $HOME` |
 | T7 | process substitution | `cat <(echo rm) -rf $HOME` | `cat echo rm -rf $HOME` |
-| T8 | base64-pipe-shell idiom | `echo cm0gLXJmICRIT01F \| base64 -d \| sh` | (separate BLOCK_RULE — not normalization) |
+| T8 | base64-pipe-shell idiom | `echo <base64 of the baseline command> \| base64 -d \| sh` | (separate BLOCK_RULE — not normalization) |
 | T9 | eval-via-variable (one-level forward flow) | `X=rm; $X -rf $HOME` | `X=rm; rm -rf $HOME` |
 
 The canonical destructive target throughout the gallery is

examples/bash-evasion-gallery/expected-findings.md

@@ -56,7 +56,7 @@ Code 2.1.98+ harness-level checks.
   in `expected-findings.md` for documentation; the run script
   only checks exit code 2, so it continues to pass after a
   rename)
-- The base64 blob in T8 (`cm0gLXJmICRIT01F`) decodes to the
-  literal command. If you change the canonical destructive
-  target away from `$HOME`, regenerate the blob with
-  `printf '<new-cmd>' | base64`
+- The base64 blob in T8 (`T8_BLOB`) is built at runtime from the
+  same fragments as the baseline command, so changing the
+  canonical destructive target updates it too. No runnable
+  base64-to-shell line is kept on disk (v8.1.3)

examples/bash-evasion-gallery/run-evasion-gallery.mjs

@@ -51,6 +51,9 @@ const RM = 'r' + 'm';
 const RF = '-' + 'rf';
 const TARGET = '$' + 'HOME';
 const PIPE_SH = '| ' + 'sh';
+// T8's blob is the canonical command base64-encoded at runtime, so no
+// runnable base64-to-shell line sits in this file (v8.1.3).
+const T8_BLOB = Buffer.from(`${RM} ${RF} ${TARGET}`).toString('base64');
 
 const CASES = [
   {
@@ -108,8 +111,8 @@ const CASES = [
   },
   {
     tag: 'T8 — base64-pipe-shell idiom',
-    // echo cm0gLXJmICRIT01F | base64 -d | sh — base64 of "rm -rf $HOME"
-    command: `echo cm0gLXJmICRIT01F | base64 -d ${PIPE_SH}`,
+    // echo <T8_BLOB>, decoded and piped into a shell
+    command: `echo ${T8_BLOB} | base64 -d ${PIPE_SH}`,
     note: 'separate BLOCK_RULE — not a normalization, but the same shape',
     expectBlock: true,
   },

tests/e2e/attack-chain.test.mjs

@@ -130,7 +130,8 @@ describe('e2e attack-chain — Stage 2: destructive Bash with evasion', () => {
   it('blocks T8 base64-pipe-shell (echo … | base64 -d | bash)', async () => {
     const result = await runHook(BASH_GUARD, {
       tool_name: 'Bash',
-      tool_input: { command: 'echo cm0gLXJmIC9ldGM= | base64 -d | bash' },
+      // Blob encoded at runtime: no runnable base64-to-shell line on disk (v8.1.3).
+      tool_input: { command: `echo ${Buffer.from('rm -rf /etc').toString('base64')} | base64 -d | bash` },
     });
     assert.equal(result.code, 2);
     assert.match(result.stderr, /base64/i);