[BUG] Bash mode does not execute commands in Bash

Environment

• Platform (select one):
• [x] Anthropic API
• [ ] AWS Bedrock
• [ ] Google Vertex AI
• [ ] Other: <!-- specify -->
• Claude CLI version: 1.0.41 (Claude Code)
• Operating System: macOS 15.5
• Terminal: fish shell with iTem 2

Bug Description

The ! command trigger in Claude Code always uses /bin/zsh for command execution, even though:

1. The user's shell is fish (or bash);
2. Claude Code was launched from fish (or bash);
3. ! is called "Bash mode" in the shortcuts.

<img width="168" alt="Image" src="https://github.com/user-attachments/assets/d5f3a607-acd8-487c-9835-20c31a2d1490" />

<img width="182" alt="Image" src="https://github.com/user-attachments/assets/09a90df1-9da5-49b3-9021-7cb68a91b45a" />

Steps to Reproduce

1. Set fish (or bash) as the default shell: chsh -s /opt/homebrew/bin/fish
2. Launch Claude Code from fish (or bash): claude
3. Use any ! command, e.g., !echo $SHELL or !echo $0
4. Observe that the output is /bin/zsh and commands are executed in zsh.

<img width="129" alt="Image" src="https://github.com/user-attachments/assets/cf7c6b77-11c1-4e61-bc48-9c077ad4e052" />

p.s. The same occurs with the Bash tool.

<img width="187" alt="Image" src="https://github.com/user-attachments/assets/08ce31ca-9dc2-470d-82fe-e35106b00b1d" />

Expected Behavior

The ! command should execute commands in either Bash as it claims or the shell from which Claude Code was launched.

Actual Behavior

• Commands are always executed using /bin/zsh
• The SHELL environment variable is set to /bin/zsh within Claude Code
• Shell-specific features and configurations from fish are not available

Additional Context

• As mentioned in https://github.com/anthropics/claude-code/issues/2358, I think we need better documentation for Bash mode behaviors.