feat: support CODEX_HOME override

[DRanger666]

Summary

• Align codex-auth with Codex's CODEX_HOME behavior.
• When CODEX_HOME is set, use it as the active Codex state root; otherwise fall back to HOME/.codex, then USERPROFILE/.codex on Windows.
• Match Codex semantics by requiring an explicit CODEX_HOME to point to an existing directory.

Notes

• Managed service definition files do not move into CODEX_HOME.
• Linux systemd --user units, macOS LaunchAgent plists, and the Windows scheduled task still live in the platform-standard per-user locations.
• The managed watcher process now keeps using the same codex_home that was active when config auto enable ran.

What Changed

• add CODEX_HOME validation and canonicalization in the Codex home resolver
• forward the active codex_home into managed watcher startup on Linux and macOS
• pass an explicit --codex-home argument to the Windows helper task
• add resolver coverage for explicit CODEX_HOME
• add e2e coverage for HOME != CODEX_HOME
• update docs to reflect the actual service-definition and watcher behavior

Validation

• zig build run -- list
• zig test src/main.zig -lc --test-filter "resolveCodexHomeFromEnv"
• zig test src/main.zig -lc --test-filter "persistent daemon watcher"
• zig test src/main.zig -lc --test-filter "mac plist"
• zig test src/main.zig -lc --test-filter "windows task"
• zig test src/main.zig -lc --test-filter "CODEX_HOME override when running login"

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commit: d3645be

[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.

[DRanger666]

I pushed a follow-up commit (8737d12) that fixes the Windows CI failure by making the test path expectation platform-aware instead of hardcoding a POSIX-style path.