Replace hardcoded "bash" shell references with platform-aware utilities so
spawn works natively from PowerShell on Windows without WSL or Git Bash.
- New shared/shell.ts: isWindows(), getLocalShell(), getInstallScriptUrl(),
getInstallCmd(), getWhichCommand() with platform override for testability
- local/local.ts: use getLocalShell() for runLocal() and interactiveSession()
- commands/run.ts: spawnScript/runScriptHeadless use getLocalShell()
- commands/update.ts: Windows downloads install.ps1, runs via PowerShell
- update-check.ts: Windows auto-update uses install.ps1; "where" replaces "which"
- shared/orchestrate.ts: PowerShell-compatible .spawnrc setup for local Windows
- Remote SSH commands unchanged — remote servers are always Linux
Closes#2726
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: L <6723574+louisgv@users.noreply.github.com>
