refactor: rename and enhance shell detection in local environment

Renamed _find_shell to _find_bash to clarify its purpose of specifically locating bash. Improved the shell detection logic to prioritize bash over the user's $SHELL, ensuring compatibility with the fence wrapper's syntax requirements. Added a backward compatibility alias for _find_shell to maintain existing imports in process_registry.py.

tools/environments/local.py

@@ -17,19 +17,19 @@ from tools.environments.base import BaseEnvironment
 _OUTPUT_FENCE = "__HERMES_FENCE_a9f7b3__"
 
 
-def _find_shell() -> str:
-    """Find the best shell for command execution.
+def _find_bash() -> str:
+    """Find bash for command execution.
 
-    On Unix: uses $SHELL, falls back to bash.
+    The fence wrapper uses bash syntax (semicolons, $?, printf), so we
+    must use bash — not the user's $SHELL which could be fish/zsh/etc.
     On Windows: uses Git Bash (bundled with Git for Windows).
-    Raises RuntimeError if no suitable shell is found on Windows.
     """
     if not _IS_WINDOWS:
         return (
-            os.environ.get("SHELL")
-            or shutil.which("bash")
+            shutil.which("bash")
             or ("/usr/bin/bash" if os.path.isfile("/usr/bin/bash") else None)
             or ("/bin/bash" if os.path.isfile("/bin/bash") else None)
+            or os.environ.get("SHELL")  # last resort: whatever they have
             or "/bin/sh"
         )
 
@@ -59,6 +59,11 @@ def _find_shell() -> str:
         "Or set HERMES_GIT_BASH_PATH to your bash.exe location."
     )
 
+
+# Backward compat — process_registry.py imports this name
+_find_shell = _find_bash
+
+
 # Noise lines emitted by interactive shells when stdin is not a terminal.
 # Used as a fallback when output fence markers are missing.
 _SHELL_NOISE_SUBSTRINGS = (
@@ -159,13 +164,11 @@ class LocalEnvironment(BaseEnvironment):
         exec_command = self._prepare_command(command)
 
         try:
-            # Use the user's shell as an interactive login shell (-lic) so
-            # that ALL rc files are sourced — including content after the
-            # interactive guard in .bashrc (case $- in *i*)..esac) where
-            # tools like nvm, pyenv, and cargo install their init scripts.
-            # -l alone isn't enough: .profile sources .bashrc, but the guard
-            # returns early because the shell isn't interactive.
-            user_shell = _find_shell()
+            # The fence wrapper uses bash syntax (semicolons, $?, printf).
+            # Always use bash for the wrapper — NOT $SHELL which could be
+            # fish, zsh, or another shell with incompatible syntax.
+            # The -lic flags source rc files so tools like nvm/pyenv work.
+            user_shell = _find_bash()
             # Wrap with output fences so we can later extract the real
             # command output and discard shell init/exit noise.
             fenced_cmd = (