Default GIT_SIGN_COMMITS to true and make it the single switch for
all signing operations. git_commit and git_tag attempt to sign when
enabled and silently retry unsigned on failure, surfacing the actual
outcome via new signed and signingWarning fields on responses.

Breaking: removed sign and forceUnsignedOnFailure inputs from
git_commit and git_tag.

Also: fixed git_pull not honoring GIT_SIGN_COMMITS (#44), fixed a
z.coerce.boolean() footgun that prevented GIT_SIGN_COMMITS=false from
actually opting out, and cleaned up dead sign? fields on merge /
rebase / cherry-pick option types.