feat: add V2 codex exec runner

[ZHOUKAILIAN]

Summary

• Add a selectable V2 codex_exec runner that invokes codex exec --json while keeping AGT-owned workflow state, prompt assembly, skill routing, and audit artifacts.
• Record Codex executor prompts/events, parse final output, commands, changed files, and token usage into existing V2 telemetry.
• Document the L1 boundary that executor protocols must stay replaceable and must not own AGT workflow state or skill routing.

Test Plan

• npm test
• npm run build
• git diff --check
• CodexExecRunner smoke: returned AGT_CODEX_RUNNER_SMOKE_OK with parsed token usage and no file changes