* feat: configurable terminal shell with Windows pwsh auto-probe
Windows previously hardcoded powershell.exe (5.1) in defaultShell(), ignoring PowerShell 7 installs. Resolve the shell in priority order: explicit shell config -> DSH_SIDEBAR_SHELL -> PATH/known-dir probe for pwsh.exe -> inbox 5.1 fallback. The same resolved shell feeds both the UI terminal tabs (PtyManager) and the model-facing terminal_* tools (AgentPtyRegistry).
defaultShell() now accepts injectable platform/env/exists options so the Windows chain is unit-testable on the ubuntu CI, which never executes win32 branches. The probe also checks ProgramW6432 for 32-bit Node, and machines without PowerShell 7 keep the 5.1 fallback.
Refs: #46
* fix: trim resolved SHELL and document ProgramW6432 in resolver contract
- POSIX SHELL is validated with trim() but was returned untrimmed; trailing whitespace could leak into the spawned executable path. Return the trimmed value like the explicit/config and Windows/env branches.
- ShellResolutionOptions.env comment now lists ProgramW6432, which the Windows probe prefers over ProgramFiles on 32-bit Node.
- Test pins the SHELL trimming behavior.
oy-g
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