feat(core): add Codex to coding agent registry

[stepandel]

Summary

Adds OpenAI's Codex CLI as an alternative coding agent backend in CODING_AGENT_REGISTRY.

Changes

• packages/core/src/coding-agent-registry.ts — New codex entry with install script, model configuration, and cliBackend definition

Details

• Install: npm install -g @openai/codex
• Config: ~/.codex/config.toml for default model
• CLI: codex exec --full-auto for one-shot execution
• Model arg: --model

Depends on: #111 (AGE-148)
Closes AGE-151

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feature/AGE-151-codex-registry

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[stepandel]

🔍 QA Review — APPROVED

All acceptance criteria for AGE-151 verified against the diff:

• ✅ CODING_AGENT_REGISTRY["codex"] exists with all required fields
• ✅ Install script installs via npm install -g @openai/codex with NVM sourcing and verification
• ✅ configureModelScript writes default model to ~/.codex/config.toml
• ✅ cliBackend configured: command=codex, args=["exec", "--full-auto"], modelArg=--model
• ✅ Follows existing claude-code entry pattern
• ✅ Build passes clean
• ✅ All 26 tests pass

Ship it 🚢

[stepandel]

🔍 QA — APPROVED by Scout

Build: ✅ clean
Tests: ✅ 45/45 passed (4 test files)
Diff reviewed against ticket acceptance criteria: ✅

Ship it 🚢

[stepandel]

🔍 QA Review — APPROVED

All acceptance criteria verified against the diff. Build passes clean. All 45 tests pass (including new constants.test.ts).

Ship it 🚢

[stepandel]

🔍 QA Review — Approved ✅

AGE-151: Codex registry entry. All fields present, build passes, 45/45 tests pass.

Tested by Scout (automated QA) — build, tests, and acceptance criteria verified.