fix(test): isolate Codex account storage

[Disaster-Terminator]

Summary

• Add a test-only data directory override for account storage in both the app crate and cockpit-core.
• Wire Codex account test guards to use that isolated data directory.
• Add regression coverage that asserts Codex account storage stays inside the test home.

Problem

Codex account tests create mock accounts such as demo@example.com, access@example.com, and auth-access@example.com. On Windows, overriding HOME and CODEX_HOME is not enough to make dirs::home_dir() resolve into the test directory, so tests can write those mock accounts into the real user data directory at .antigravity_cockpit/codex_accounts.json.

This keeps the production path unchanged and only enables the override for test builds.

Test Plan

• cargo test -p cockpit-core --lib test_env_guard_redirects_codex_account_storage
• cargo test -p cockpit-tools --lib test_env_guard_redirects_codex_account_storage
• cargo test -p cockpit-core --lib upsert_
• cargo test -p cockpit-tools --lib upsert_
• cargo test -p cockpit-core --lib modules::codex_account::tests
• cargo test -p cockpit-tools --lib modules::codex_account::tests
• git diff --check

Note: cargo fmt --check currently reports pre-existing formatting drift in unrelated files outside this PR.