fix(installer): honor CODEX_HOME for the Codex global install (#1627)

[maxmilian]

Addresses the CODEX_HOME half of #1627.

Codex resolves its user layer from CODEX_HOME and only falls back to ~/.codex. The Codex target hardcoded the fallback, so a user running Codex against a custom profile got a correct install into a directory Codex never reads — the [mcp_servers.codegraph] entry, the AGENTS.md block, and detect() all pointed at the default profile. Nothing errors; CodeGraph simply never loads.

Resolution now mirrors what the copilot-cli target already does for COPILOT_HOME:

const override = process.env.CODEX_HOME;
if (override && override.trim().length > 0) return override;
return path.join(os.homedir(), '.codex');

Only the user layer moves. The project layer from #1531 stays anchored to the project — .codex/config.toml beside the repo is not a profile artifact — and there is a test pinning that.

Scope: this is deliberately half of #1627

The issue names two variables. The CLAUDE_CONFIG_DIR half is already handled by #1029 (open since June, rebased onto current main on Aug 25, waiting on review) — it was opened before this issue existed, so the two aren't linked in either direction. This PR does not touch targets/claude.ts, so the two don't collide. Landing both closes #1627.

Tests

Four cases in a new describe, following the shape of the existing opencode XDG config path (#535) block:

• global install writes to $CODEX_HOME and nothing lands in ~/.codex — fails before the change
• detect() and uninstall() follow $CODEX_HOME too — fails before the change
• unset or blank CODEX_HOME still falls back to ~/.codex — green before and after, guarding the fallback
• local install is unaffected by CODEX_HOME (Codex CLI now supports project-local config (.codex/config.toml) — CodexTarget.supportsLocation('local') is stale #1531) — green before and after

One extra fix in the harness: setHome() clears HERMES_HOME and COPILOT_HOME but not CODEX_HOME. Now that the target reads it, the existing codex tests would fail on any machine that has the variable exported, so it is cleared and restored alongside the other two.

Full suite: 178 files / 3056 tests passing (3052 on main plus these four). npm run build clean.

Reported by @seanchann.