## Why
On Windows, Codex runs shell commands through a top-level
`powershell.exe -NoProfile -Command ...` wrapper. `execpolicy` was
matching that wrapper instead of the inner command, so prefix rules like
`["git", "push"]` did not fire for PowerShell-wrapped commands even
though the same normalization already happens for `bash -lc` on Unix.
This change makes the Windows shell wrapper transparent to rule matching
while preserving the existing Windows unmatched-command safelist and
dangerous-command heuristics.
## What changed
- add `parse_powershell_command_plain_commands()` in
`shell-command/src/powershell.rs` to unwrap the top-level PowerShell
`-Command` body with `extract_powershell_command()` and parse it with
the existing PowerShell AST parser
- update `core/src/exec_policy.rs` so `commands_for_exec_policy()`
treats top-level PowerShell wrappers like `bash -lc` and evaluates rules
against the parsed inner commands
- carry a small `ExecPolicyCommandOrigin` through unmatched-command
evaluation and expose `is_safe_powershell_words()` /
`is_dangerous_powershell_words()` so Windows safelist and
dangerous-command checks still work after unwrap
- add Windows-focused tests for wrapped PowerShell prompt/allow matches,
wrapper parsing, and unmatched safe/dangerous inner commands, and
re-enable the end-to-end `execpolicy_blocks_shell_invocation` test on
Windows
## Testing
- `cargo test -p codex-shell-command`
