Add blocking PermissionRequest hook for external approval UIs

The hooks system (UserPromptSubmit, SessionStart, Stop) is great for observability. What's missing is a blocking hook for tool approval - letting external processes approve/deny commands before execution.

Use case: Desktop overlays, IDE extensions, and CI/CD pipelines that want to handle approval flows outside the TUI.

We hit this building Masko Code (desktop overlay for Codex + Claude Code) - currently we can ingest Codex session logs but can't send approval responses back.

Proposed contract (similar to Claude Code's):

• Hook fires when Codex needs approval for a command
• Blocks until hook script exits
• Exit 0 = allow, exit 2 = deny
• Payload includes: command, tool name, prefix_rule, sandbox permissions
• Hook output can include allow_persistent to auto-approve the prefix

PreToolUse (#15211) is close but is bash-only and deny-only. This would cover all tool types with bidirectional responses.

Related: #14882, #13498, #15211