core: shell tests fail on Windows when launched from Git Bash (ambient SHELL env) #54104

Description

packages/core shell tests branch command syntax on process.platform === "win32" and hard-code PowerShell (e.g. test/tool-shell.test.ts), but shell selection (src/shell/select.ts, environment()) prefers process.env.SHELL whenever it is set. Git Bash exports SHELL=/usr/bin/bash, so running the core suite from Git Bash makes the shell tool select bash and every platform-branched command fails with bash syntax errors:

/usr/bin/bash: -c: line 1: syntax error near unexpected token `$env:OPENCODE_SESSION_ENV_TEST'

CI stays green because its bash steps do not propagate SHELL into the test process, so Windows CI silently runs PowerShell while a Git Bash developer machine runs bash. script/test.ts also forwards SHELL (its env filter only strips provider credentials), so the project runner reproduces this too.

Suggested fix: normalize SHELL in test/preload.ts on win32 so unit tests exercise the platform default shell, matching the PowerShell syntax the tests already assert. PR incoming.

Steps to reproduce

1. On Windows, open Git Bash in a v2 checkout with dependencies installed.
2. cd packages/core && SHELL=/usr/bin/bash bun run test test/tool-shell.test.ts
3. ShellTool tests fail with bash syntax errors; the same command from PowerShell (where SHELL is unset) passes.

Operating System

Windows 11 (10.0.26200)

Terminal

Git Bash (MINGW64)

OpenCode version

v2 branch @ 2f3ba04