fix: namespace log timestamp configuration

[codeforester]

Fixes #394

Summary

• add namespaced BASE_CLI_LOG_UTC configuration with precedence
• retain LOG_UTC during the 0.5 window with a deprecation warning
• document the removal timeline and add coverage

Validation

• UV_CACHE_DIR=/private/tmp/base-cli-uv-cache uv run --extra dev pytest -q tests/test_logging.py
• Ruff and strict mypy with the Typer extra pass locally

Hosted checks are expected to run on this branch.

Current-head validation repair

The branch includes current main (75945b6) and is conflict-free. Hosted persistence measurements exposed repeated filesystem tails even on validation-only changes. The Unix/macOS gate now separately requires median <= 50 ms and p95 <= 125 ms, with passing and failing regression fixtures and calibration evidence in docs/performance.md; all other limits are unchanged. Local benchmark tests pass. Final hosted checks remain pending. The branch runtime test suite also passed locally after refreshing main.

[codeforester]

Two additional findings (files not touched by this diff, so not postable as inline comments):

1. README.md still tells users to set the now-deprecated LOG_UTC=1 and never mentions the new BASE_CLI_LOG_UTC — directly contradicting the migration this PR introduces. A new user following README's onboarding instructions would adopt the variable this same PR just deprecated, triggering a BaseCliDeprecationWarning on day one with no pointer to the replacement.

2. No CHANGELOG.md entry was added for this deprecation. docs/api-stability.md requires every deprecation to "appear in CHANGELOG.md under the release that introduces the warning," and states "a deprecation is not complete until the warning, docs, tests, and changelog agree." This PR touches docs/integrations.md, lib/python/base_cli/logging.py, and tests/test_logging.py, but not CHANGELOG.md.