refactor(core): drop unused base.sh helpers with no production callers

Removed five helpers in lib/core/base.sh that had test coverage but zero
production callers across mole/install.sh/lib/bin/scripts:

- is_sip_enabled (defensive SIP check, never consulted by any flow)
- is_darwin_ge (wrapper around get_darwin_major; nobody gated on version)
- get_invoking_user (user-name variant; get_invoking_uid is the live one)
- get_brand_name (56 lines of zh/en app-name mapping; i18n never shipped)
- get_mole_temp_root (4-line wrapper; prepare_mole_tmpdir is the real entry)

Tests updated: rewrote temp-root tests to exercise prepare_mole_tmpdir,
dropped the standalone unit tests for the deleted helpers, and removed
12 preventive is_sip_enabled() mocks from clean_system_maintenance.bats.