Git subprocess environment hardened, with explicit control over terminal prompts

What

• New, frozen sets of environment variables are applied to internal git invocations: GIT_ALLOW_PROTOCOL="none", GIT_NO_LAZY_FETCH="1", GIT_NO_REPLACE_OBJECTS="1", GIT_TERMINAL_PROMPT="0", and, unless SSH is explicitly allowed, GIT_SSH_COMMAND="false".
• The internal git command-construction helper gained a new allowGitTerminalPrompt option; unless a call explicitly allows it, GIT_TERMINAL_PROMPT is stripped from the environment passed to the git subprocess, whereas before it could pass through unfiltered.
• Git push calls used internally (for worktree operations and plugin tag creation) now explicitly set allowGitTerminalPrompt: disabled for background worktree pushes, enabled for interactive plugin tag pushes.

Why

This reduces the risk of internal git commands silently reaching the network, replacing objects, or hanging on an interactive credential prompt, while still allowing prompts in the specific cases where a human is present to answer them.