[codex] add codex cli execution backend

[jonaswre]

Summary

Adds OpenAI Codex CLI as a first-class Pilot execution backend alongside Claude Code, Qwen Code, and OpenCode.

Changes

• Adds codex-cli backend type and executor.codex_cli configuration.
• Implements a Codex CLI subprocess backend using codex exec --json.
• Normalizes Codex JSONL events into Pilot backend events.
• Supports model routing, reasoning effort mapping, session resume, heartbeat/watchdog handling, stderr diagnostics, RSS telemetry, and backend error classification.
• Wires Codex CLI into backend factory, preflight, doctor health checks, pilot backend commands, and onboarding.
• Updates example config and docs for prerequisites, configuration, and backend comparison.
• Adds unit and fake-subprocess coverage for Codex command construction, event parsing, error classification, factory/config behavior, CLI backend UX, and preflight mapping.

Validation

env -u ANTHROPIC_API_KEY -u ANTHROPIC_AUTH_TOKEN -u CLAUDE_CODE_OAUTH_TOKEN -u OPENAI_API_KEY -u CODEX_API_KEY \
  go test ./internal/executor ./cmd/pilot ./internal/config ./internal/health ./internal/wiring

Passed locally.