fix: gate remote branch deletion pushed as an empty refspec

[Arakiss]

git push origin :main deletes the remote branch. Today that command is allowed outright, while an ordinary git push origin main requires a signed picto.

Cause. bash-git-push captures the whole refspec into ref, so :main emits git.push:refs/heads/:main. No branch gate matches that string and the call falls through to allow.

Fix. A mapper for the empty-source refspec that emits the destination branch, so the existing branch gates apply, plus a dedicated git.push.delete capability. The new gate-remote-branch-delete rule covers every branch, not only protected ones: deleting a colleague's feature branch destroys shared work even where pushing to it is allowed.

Mappers accumulate rather than short-circuit (mapper.rs:154), so adding one does not alter existing rules.

Verified against a copy of a real policy tree, comparing the shipped binary with the built one:

command	before	after
git push origin :main	allow	ask_picto
git push origin :dev	allow	ask_picto
git push origin :feat/algo	allow	ask_picto
git push origin main	ask_picto	ask_picto
git push --force origin main	ask_picto	ask_picto
git push origin feat/x	allow	allow

Two regression fixtures added to examples/policy-fixtures.yaml, which CI already runs. cargo fmt --check clean, cargo clippy --all-targets -- -D warnings clean, 155 tests pass.