Feature Request: Configurable Shell for AI Agent & Tasks

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Description:
Currently, Zed allows configuring the default terminal shell via settings.json (e.g., "terminal.shell.program": "C:\path\to\bash.exe"), but the AI Agent and task runner continue to execute commands in the system’s default shell (PowerShell on Windows).

This creates inconsistency:

The integrated terminal respects my chosen shell (MSYS2 Bash, Git Bash, WSL, etc.).
The AI Agent and tasks ignore this setting, defaulting to PowerShell or cmd.exe on Windows.

Why this matters:

Developers often rely on specific environments (MSYS2, Git Bash, WSL, custom shells) for PATH setup, toolchains, or package managers. Having the AI Agent and tasks run in a different shell leads to broken commands, inconsistent behavior, and extra manual setup. Aligning all shells (terminal, agent, tasks) would make Zed’s developer experience seamless and predictable.

Proposed Solution:
Introduce a new configuration key, for example:

"agent": {
  "shell": {
    "program": "C:\\Develop\\msys2\\ucrt64_bash.bat",
    "args": []
  }
},
"tasks": {
  "default_shell": {
    "program": "C:\\Develop\\msys2\\ucrt64_bash.bat",
    "args": []
  }
}

Alternatively, reuse the existing terminal.shell setting as the default for tasks and the agent unless overridden.

Benefits:

Consistent shell behavior across terminal, AI Agent, and task runner. Easier integration with non-PowerShell/Unix environments (MSYS2, Cygwin, WSL, Git Bash, etc.). Less friction for developers on Windows who rely on Unix-like shells.

Environment:

OS: Windows 10/11

Shell: MSYS2 Bash (ucrt64_bash.bat)

Zed version: 0.201.8