feat: support CODEX_HOME override

[DRanger666]

Title:

feat: support CODEX_HOME override

Description:

Summary

This PR adds CODEX_HOME support to codex-auth.

Current behavior only resolves the Codex state root from:

• HOME/.codex
• USERPROFILE/.codex on Windows

After this change, resolution order becomes:

1. CODEX_HOME when set to a non-empty path
2. HOME/.codex
3. USERPROFILE/.codex on Windows

This keeps existing behavior intact while allowing users to keep
Codex auth, registry, config, and session state in a custom
location.

What changed

• added CODEX_HOME precedence in the Codex home resolver
• kept existing fallback behavior for HOME/.codex and
  USERPROFILE/.codex
• updated docs to describe the new storage-root resolution order
• updated e2e tests to set an explicit test-local CODEX_HOME so
  they do not depend on ambient user environment state
• added resolver tests covering:
  • CODEX_HOME override
  • empty CODEX_HOME fallback to HOME
  • USERPROFILE fallback when HOME is unset

Why

Some users run Codex with a custom CODEX_HOME and expect
related tooling to operate on the same state root. Without this
change, codex-auth always targets the default home-based path,
which makes it diverge from the active Codex environment.

Validation

Validated with the project test suite using Zig 0.15.1.

• zig build
• zig test src/main.zig -lc

Result:

• all 273 tests passed

Compatibility

This change is backward-compatible:

• existing users without CODEX_HOME set still use HOME/.codex
• Windows fallback behavior remains in place
• test coverage was updated to preserve and verify the old
  behavior alongside the new override behavior

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commit: 82ebeaa

[Loongphy]

Thanks for your contribution.

It was supported in early versions but was removed in a later release. It's meaningful and also convenient for testing, let's restore it.