Add codex exec review: headless code review

What feature would you like to see?

Why
Enable CI/automation without requiring the interactive TUI.

What
Introduce codex exec review command with presets:

• uncommitted
• base-branch <branch>
• commit <hash> [--subject <subject>]
• custom [PROMPT|-]

How
Reuse existing review flow and emit lifecycle events.
Surface a structured JSON result for machine parsing; keep human mode to final review text on stdout.

Examples
codex exec review uncommitted
codex exec review base-branch main
codex exec review commit d5853d9 --subject "Fix sandbox"
codex exec review custom "Check for security vulnerabilities"

Acceptance
All presets run end-to-end with TUI-equivalent output.
Structured result is CI-friendly and easy to parse; human mode prints only final review content.
Behavior matches current codex review logic.

Note: Implementation ready — opening this issue for visibility and review. Happy to open PR if maintainers are open to this feature.

Additional information

_No response_