Add Codex compatibility with runtime-agnostic agent/session handling

[mexicorea]

Summary

This PR adds first-pass Codex compatibility and refactors agent/session handling to be runtime-agnostic (Claude + Codex), while keeping existing Claude behavior intact.

What Changed

• Added runtime abstraction (claude | codex) for:

  • terminal launch command
  • session root resolution
  • terminal naming

• Made agent lifecycle/runtime state explicit (runtime, pending session linking).

• Added Codex session discovery/linking from ~/.codex/sessions/**/*.jsonl using session_meta.payload.cwd.

• Added Codex transcript parsing:

  • function_call -> tool start
  • function_call_output -> tool done
  • final_answer / task_complete -> waiting

• Isolated Codex-specific logic into:

  • src/codex/session.ts
  • src/codex/transcript.ts

• Added shared JSONL file utility:

  • src/sessionFiles.ts

• Added Settings support for default runtime (Claude/Codex) + persistence.

  • 

• Updated CLAUDE.md to reflect the new architecture and message flow.

Why

Claude and Codex have fundamentally different session/transcript behaviors. This change introduces a clean runtime split so both can be supported safely without embedding Codex-specific logic across the core flow.

Validation

• Manual smoke test in Extension Development Host:
  • + Agent works for both Claude and Codex
  • tool/waiting status updates are emitted correctly
  • runtime setting persists and affects new launches