E2E coverage for agent CLIs on native Windows

**Context** (PR #33 self-review follow-up): the agent CLI path (Claude Code / Codex / Qoder) is launched via a bash-based Node/nvm bootstrap, so \`internal/agent\` on native Windows currently has zero e2e coverage. The Windows e2e job (\`E2E (none runtime, Windows)\`) only exercises mock-engine + script-judge.

**Proposal**:
- Either: rework the Node/nvm bootstrap to run under PowerShell, then enable the existing e2e suite on Windows.
- Or: add a Windows-WSL job that boots WSL, installs the agent CLIs, and runs the existing full e2e — covers the recommended workflow.
- Or (interim): add a Windows-host job that requires Git Bash pre-installed and runs full e2e through bash — covers the documented limitation path.

Currently the recommended workflow ("use WSL2") has no CI verification.