fix(tests): isolate Codex home from user state

[aaiyer]

What does this PR do?

Completes the test-state isolation added in #1067. The runner replaced HOME and cleared CBM_CACHE_DIR, but it still inherited the invoking process's CODEX_HOME. When that directory existed, CLI fixtures detected the developer's real Codex installation and repeatedly installed and uninstalled MCP config, AGENTS.md content, skills, and Scout/Verify/Auditor profiles there.

A sentinel run on exact upstream/main e678b2b reproduced thousands of accesses and explicit write syscalls under the inherited CODEX_HOME, left an extra skills directory, and failed the install-plan no-mutation fixture because the inherited override shadowed its temporary .codex directory.

Fix

• set CODEX_HOME to the private test HOME's .codex path before normal suites run;
• fail test-home setup instead of continuing if the environment isolation cannot be established;
• add a regression that binds CODEX_HOME to the private test HOME;
• explicitly unset and restore CODEX_HOME in the two tests that intentionally exercise the home-directory fallback.

Worker and socket-probe modes still exit before normal suite setup, as before.

Validation

• ASan + UBSan test runner build with warnings as errors: passed;
• make -f Makefile.cbm test-focused TEST_SUITES=cli: 223 passed;
• inherited fresh CODEX_HOME sentinel under strace: 0 sentinel-path syscalls after the fix, identical file hashes and directory inventory;
• bash scripts/check-no-test-skips.sh: passed;
• bash scripts/security-audit.sh: passed with existing review notices only;
• bash scripts/security-vendored.sh: passed (75 files);
• git diff --check: passed;
• DCO check: passed.

Local CI lint was attempted but cppcheck was unavailable; clang-format and clang-tidy are also not installed in this environment. GitHub CI remains the lint authority.

Coordination

#1149 overlaps the same install-plan fallback test for a separate fixture-lifetime issue. Whichever PR lands second will need to rebase and preserve both intents: free the receipt JSON on every validation-failure path, and explicitly isolate CODEX_HOME while exercising the fallback path.

Scope

Only tests/test_main.c and tests/test_cli.c change. There is no production behavior or dependency change.