Code Review: parametrize-base-branch

SPEC coverage

Req	Status
R1 — _gate_worktree_drift reads base_branch	✓ `_base_branch(cfg)` in git diff argv
R2 — Helper: None→main, empty→main+WARN, non-str→str+WARN, explicit→explicit	✓
R3 — Bad-revision still WARNING-skip	✓ unchanged
R4 — Template includes base_branch	✓
R5 — No new deps	✓

No impact on loop-runner.py. Only status.py and the template.

PASS.