Agent abstraction Phase C: ship OpenAI Codex MVP

[dhilgaertner]

Summary

Phase C of the agent-abstraction work scoped by #150. Implement OpenAICodexAgent in a new CrowCodex Swift package, enable the per-session selector for Codex, and deliver end-to-end Codex session creation.

Context

Full design in docs/agent-abstraction-spec.md §3, §7, §12 Phase C.

Codex has native hooks (feature-flagged), an AGENTS.md context file analog, and a notify command as a fallback signal source. The spec calls for a two-tier state-signal approach: Codex hooks as Tier 1 (when codex_hooks = true) and notify as the always-on Tier 2 fallback.

Checklist

• Create Packages/CrowCodex/ Swift package (symmetric to CrowClaude)
• OpenAICodexAgent with binary resolver: /opt/homebrew/bin/codex, /usr/local/bin/codex, ~/.local/bin/codex, plus a which codex PATH fallback (see spec §13.4)
• Codex prompt template with a "plan-first" preamble (no /plan, no dangerouslyDisableSandbox directives)
• CodexHookConfigWriter writing .codex/hooks.json for the 5 supported events (SessionStart, PreToolUse, PostToolUse, UserPromptSubmit, Stop); each event invokes crow hook-event --agent codex --event <Name>
• CodexSignalSource Tier 1 (hooks) + Tier 2 (notify fallback)
• Scaffolder writes .codex/config.toml (with [features] codex_hooks = true and notify = ["crow", "codex-notify", "--session", "<uuid>"]), AGENTS.md, and a .agents/skills/… placeholder
• crow hook-event CLI gains --agent <kind> flag (default claudeCode); new crow codex-notify subcommand (or reuse of crow hook-event --agent codex --event Notification)
• Enable Codex option in the New Session selector; gate behind codex binary presence on the machine
• Codex-flavored crow-workspace skill authored, or explicitly documented as Claude-only for this phase
• supportsRemoteControl = false for Codex; UI hides the remote-control badge for Codex sessions
• End-to-end verification: create a Codex session, submit a prompt, observe AgentActivityState transitioning .idle → .working → .done via the notify fallback. If the user enables codex_hooks, confirm the hook path produces the same transitions with finer resolution. Claude sessions continue to behave identically.

Non-goals

• Codex remote control (Phase D)
• Per-agent notification sounds / icons (Phase D)
• Mid-session agent switching (out of scope per spec §14)

Parent: #150 · Blocked by: #166 (Phase A), #167 (Phase B) · Blocks: #169 (Phase D)