Fix Windows terminal shell and Python resolution

[luckeyfaraday]

Summary

• prefer PowerShell 7 (pwsh.exe) for Windows panes, with Windows PowerShell fallback
• use the same resolver for embedded shells, agent panes, and the dedicated Codex terminal
• remove inherited Python virtual-environment state from terminal processes
• put CONTEXT_WORKSPACE_PYTHON first on PATH so Athena's configured interpreter wins
• add regression coverage for shell fallback and Python environment resolution

Verification

• based on current main / Athena 0.1.12
• npm run test:electron (231 tests: 226 passed, 5 platform skips, 0 failures)
• git diff --check

Closes #194

Supersedes closed PR #195, which was mistakenly prepared from a stale checkout.