[FEATURE]:Make `bash` tool respect `shell` config or add env var override

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Summary

The built-in bash tool on Windows is hardcoded to use cmd.exe /c, ignoring the shell field in opencode.json (e.g., "shell": "pwsh"). Please make the tool configurable.

Current Behavior

• opencode.json has "shell": "pwsh"
• But the AI's bash tool always uses cmd.exe /c on Windows
• Config value is only used for TUI terminal, MCP server spawn, shell completions, etc. — not for the bash tool

Expected Behavior

One of:

1. Read shell config — bash tool uses the configured shell (pwsh, bash, nu, etc.)
2. Add env var override — e.g., OPENCODE_BASH_SHELL=pwsh or OPENCODE_SHELL=pwsh
3. Add CLI flag — --shell=pwsh that affects the tool

Rationale

• PowerShell 7+ (pwsh) is widely available on Windows and is the modern default
• Users explicitly configure shell: "pwsh" expecting it to apply everywhere
• Current workaround requires every command to be wrapped: pwsh -c "..." — verbose and error-prone
• Cross-platform consistency: on Unix the tool uses sh -c (respects $SHELL indirectly), Windows should respect config too

Workaround Today

# Every single command must be wrapped
pwsh -c "Get-ChildItem"
pwsh -c "git status"

Suggested Implementation

// packages/opencode/src/tool/bash.ts
const shell = config.shell ?? process.env.OPENCODE_BASH_SHELL ?? (isWindows ? "cmd.exe" : "sh");
const args = isWindows ? ["/c"] : ["-c"];

Environment

• opencode version: latest
• OS: Windows 10/11
• Shell: PowerShell 7.6.3 (pwsh)

Related: The shell config already exists and works for other subsystems — just not the bash tool. This feels like an oversight.