feat(codex): support extra env vars

[yzlu0917]

Summary

• add agent.options.env support to the Codex agent
• merge option env, provider env, and session env when starting Codex sessions
• add regression tests covering env parsing, env merging, and env propagation to the
  codex subprocess

Why

Codex sessions already inherit provider-specific environment variables, but there was
no direct way to set extra runtime env for users relying on local Codex login instead
of a configured provider. This is especially useful for proxy variables in auto- edit / full-auto sandboxed workflows.

Example

[projects.agent]
type = "codex"

[projects.agent.options]
work_dir = "/path/to/project"
mode = "auto-edit"
env = { HTTP_PROXY = "http://127.0.0.1:7890", HTTPS_PROXY = "http://127.0.0.1:7890",
ALL_PROXY = "socks5://127.0.0.1:1080", NO_PROXY = "127.0.0.1,localhost" }

## Validation

- checked the patch with git diff --check
- ran go test ./agent/codex/...

[chenhg5]

LGTM. Practical feature for Codex users.

Review summary:

• ✅ Supports agent.options.env for extra environment variables
• ✅ Merges option env, provider env, and session env correctly
• ✅ Tests for env parsing and propagation
• ✅ CI passes

Useful for proxy variables in sandboxed workflows.