fix Windows terminal capability detection

[fengju0213]

Summary

• Detect native Windows shells when deciding whether the backend can register terminal tools.
• Keep existing bash/sh detection for POSIX environments.
• Add regression coverage for Windows PowerShell and POSIX sh detection.

Root Cause

BrainCapabilities only checked for bash before enabling the terminal hand. On Windows desktop installs, users can have PowerShell or cmd available without bash, so the backend marked terminal as unavailable and skipped TerminalToolkit. The multi-modal agent could then call shell_exec from its prompt even though the tool was not registered, producing KeyError: 'shell_exec'.

Validation

• uv run pytest tests/app/hands/test_capabilities.py
• uv run ruff check app/hands/capabilities.py tests/app/hands/test_capabilities.py
• uv run ruff format --check app/hands/capabilities.py tests/app/hands/test_capabilities.py
• backend pre-commit hooks during commit