fix: resolve default Codex home from USERPROFILE on Windows

[Wangnov]

Problem

On Windows, when neither CODEX_HOME nor --codex-home is set, codex-threadripper derived the default Codex home as $HOME/.codex, reading only the HOME environment variable.

Codex itself locates its home on Windows via dirs::home_dir(), which resolves from USERPROFILE and ignores HOME. When a process injects HOME into the environment (e.g. an app pointing it at %APPDATA%\<app>), the two diverge: Codex writes its DB under C:\Users\<user>\.codex, while threadripper looked under %APPDATA%\<app>\.codex and failed with:

no Codex state database found under C:\Users\...\AppData\Roaming\SPB_16.6\.codex (looked at state_5.sqlite and sqlite/state_5.sqlite) — run Codex at least once to create it

Fix

Introduce a pure home_base_from_env(userprofile, home, is_windows) that resolves the default home base:

• Windows: prefer USERPROFILE, fall back to HOME (for shells like Git Bash/MSYS that only set HOME)
• Unix: keep HOME
• empty values are skipped

The call site passes the platform via cfg!(windows), so both branches compile and are unit-tested on every platform.

Testing

• New home_base_prefers_userprofile_on_windows covers: Windows prefers USERPROFILE (ignoring a hijacked HOME), falls back to HOME when USERPROFILE is missing/empty, Unix uses only HOME, plus an end-to-end repro of the reported issue.
• Local cargo fmt --check / cargo test --locked (73 passed) / cargo clippy --locked --all-targets -- -D warnings all green.
• codex exec review --commit HEAD: no actionable regressions.

Compatibility

• Unix behavior unchanged.
• On Windows where HOME == USERPROFILE (the common case) the result is unchanged; only when HOME is overridden does it now follow USERPROFILE, matching Codex.
• Explicit --codex-home / CODEX_HOME precedence is unchanged.

[chatgpt-codex-connector]

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