fix: on-start/status honor the git-repo gate

[winjer]

Problem

When CODE_TRACE_REQUIRE_GIT_REPO is on (the default), the Stop hook correctly refuses to trace outside a git repo. But the SessionStart reminder and code-trace status ignored the gate and still told the user "tracing ENABLED" — false reassurance in exactly the places the gate exists for.

Fix

• on_start(): prints tracing inactive (not in a git repository) when the gate blocks tracing.
• status(): same check, same wording.

Tests

• on_start_reports_inactive_outside_git_repo
• on_start_reports_enabled_inside_git_repo
• Extended local TestEnv in cli_test.rs with with_env() so individual tests can re-enable the gate.

All 186 tests pass.