fix(tools): keep non-bash -c invocations covered by the shell guard

The _bash_exec_payload delegation rejected short-option bundles with
letters outside bash's alphabet, so 'zsh -yc', 'dash -Vc' and 'ksh -Gc'
scripts stopped being scanned — a fail-open regression for shells the
guard's _SHELL_EXECUTABLES explicitly covers. Try the bash grammar
first (catches operand-hidden -c), then fall back to the permissive
positional scan; a block-guard fails closed.

tests/tools/test_self_repo_guard.py

@@ -84,6 +84,9 @@ class TestBlocksMutationsInSourceRepo:
             "bash -lc 'git switch main'",
             "bash -o pipefail -c 'git checkout main'",
             "bash +O extglob -c 'git checkout main'",
+            "zsh -yc 'git checkout main'",
+            "dash -Vc 'git checkout main'",
+            "ksh -Gc 'git checkout main'",
         ],
     )
     def test_wrappers_and_nested_shells(self, repo, command):

tools/self_repo_guard.py

@@ -331,14 +331,26 @@ def _cd_target(executable: str, args: list[str], cwd: Path) -> Path | None:
 def _shell_script_arg(args: list[str]) -> str | None:
     """Return the script string owned by a shell's ``-c``, if present.
 
-    Delegates to approval.py's ``_bash_exec_payload``, which parses bash's
-    real option grammar (``-O/-o`` consume the next argument, short-option
-    bundles, ``--init-file``/``--rcfile``). A naive "leading option containing
-    'c'" scan fails open on ``bash -o pipefail -c '<script>'`` — the ``-o``
-    operand hides the ``-c`` and the script is never scanned.
+    Tries approval.py's ``_bash_exec_payload`` first: it parses bash's real
+    option grammar (``-O/-o`` consume the next argument, short-option
+    bundles, ``--init-file``/``--rcfile``), catching payloads a naive scan
+    misses — ``bash -o pipefail -c '<script>'`` hides the ``-c`` behind an
+    operand. When it finds no ``-c``, fall back to the permissive positional
+    scan: ``_SHELL_EXECUTABLES`` also covers zsh/dash/ksh, whose option
+    letters (``zsh -yc``, ``dash -Vc``) fall outside bash's alphabet and
+    would otherwise make this block-guard fail open.
     """
     has_c, payload = _bash_exec_payload(args)
-    return payload if has_c else None
+    if has_c:
+        return payload
+    for index, arg in enumerate(args):
+        if arg == "--":
+            break
+        if arg.startswith("-") and "c" in arg[1:]:
+            return args[index + 1] if index + 1 < len(args) else None
+        if not arg.startswith("-"):
+            break
+    return None
 
 
 def _heredoc_specs(line: str) -> list[_Heredoc]: