Remove process-global cwd mutation from base_cli.testing.invoke

[codeforester]

Problem

Validated from Claude third-pass finding [87].

lib/python/base_cli/testing.py::invoke() changes process-global working directory with os.chdir(cwd_path), calls CliRunner.invoke(...), and then changes back in a finally block. This works for serial tests, but os.chdir() is process-global and can race when helper invocations run concurrently in the same Python process, such as with pytest-xdist style patterns or future threaded test harnesses.

Desired outcome

Make base_cli.testing.invoke() avoid process-global cwd mutation while preserving its current test ergonomics.

Scope

• Prefer a Click-supported isolated cwd mechanism or another local-only strategy.
• Preserve support for writing a base_manifest.yaml fixture when manifest= is supplied.
• Keep stdout/stderr separation behavior intact.
• Add a regression test for cwd isolation.

Acceptance criteria

• invoke(..., cwd=...) no longer calls os.chdir() around the invocation, or the remaining use is isolated and safe for concurrent calls.
• Existing base_cli testing helpers continue to work.
• Tests cover cwd behavior without relying on global process state.

Classification

Review finding taxonomy: test helper robustness.