Support additional AI CLI backends (OpenAI Codex CLI, Gemini CLI, etc.)

[djdarcy]

Summary

Support additional AI CLI backends beyond Claude Code CLI for the organize step, starting with OpenAI's Codex CLI (codex). Users should be able to choose their preferred AI tool via --backend.

Context

Currently the organize command supports two backends:

• --backend claude — Invokes Claude Code CLI (claude) in non-interactive mode
• --backend prompt-only — Saves the prompt to _organize_prompt.md without running any AI
  OpenAI has released Codex CLI (codex), a command-line AI tool with file-read capabilities similar to Claude Code. Other AI CLI tools (Google Gemini CLI, etc.) are also emerging. Supporting multiple backends would:
• Allow users who don't have Claude Code CLI installed (or prefer OpenAI) to still use the organize step
• Reduce hard dependency on a single vendor's CLI
• Make it easier to add future AI CLI tools as they mature
  The current backend logic lives in organizer.py (find_claude_cli, invoke_claude_cli) and cli.py (--backend option on organize and run commands).

Proposed Changes

• Add --backend codex option to organize and run commands alongside claude and prompt-only
• Add find_codex_cli() in organizer.py to locate the codex / codex.exe binary
• Add invoke_codex_cli() in organizer.py mapping prompt and file-read permissions to Codex CLI flags
• Extract a shared invoke_ai_cli() abstraction (or a simple dispatch table) to reduce duplication between invoke_claude_cli and invoke_codex_cli
• Add --backend auto as a future-friendly option that tries Claude first, then Codex, then falls back to prompt-only
• Update README: add Codex CLI installation instructions alongside Claude Code CLI

Backend Comparison

Feature	Claude Code CLI	OpenAI Codex CLI
Non-interactive flag	-p "<prompt>"	TBD — research required
Restrict to Read tool	--allowedTools "Read,Grep"	TBD
Output format	--output-format text	TBD
Install URL	https://claude.ai/claude-code	https://github.com/openai/codex

Tasks

• Research Codex CLI invocation flags for non-interactive/headless mode
• Research how Codex CLI restricts file access (equivalent of --allowedTools Read)
• Add find_codex_cli() to organizer.py
• Add invoke_codex_cli() to organizer.py
• Update --backend choice in cli.py to include codex (and optionally auto)
• Add dispatch logic in organize command to route to the correct invoke_* function
• Update README with Codex CLI install instructions and --backend codex usage example
• Test --backend codex end-to-end with a real extraction output