docs(git): clarify unconfined default scope and repository confinement options in README and CLI #5230

Server:

git (mcp-server-git)

Server Version / Environment:

• Version: 2026.10.19 (Python SDK >=2.3,<3)
• Transport: stdio
• OS: Linux / macOS / Windows

Description & Motivation:

In mcp-server-git, tools accept repo_path (e.g., git_status, git_diff, git_commit, etc.).
When --repository <path> is passed at startup, validate_repo_path() rejects any tool calls outside the allowed repository root.

However, when launched without --repository (such as in standard client configurations like uvx mcp-server-git), allowed_repository defaults to None. In this mode, validate_repo_path() is a no-op, allowing any tool call to access and mutate any reachable Git repository across the host filesystem under the running user's process privileges (as pinned by test_without_repository_any_repository_is_reachable under #4550).

While this multi-repository reach is an intentional architectural capability, it is not prominently communicated to users in either the CLI --help or README.md. Users configuring the server may assume it is scoped only to the workspace or current directory.

Prompt Used to Generate the Change:

In src/git, clarify that omitting --repository leaves the server in unconfined mode where all reachable repositories on the filesystem are accessible to tool calls. 

1. In src/git/README.md, add an official GitHub alert (> [!WARNING]) under Configuration explaining that without --repository, every reachable Git repository is in scope, and describe how to confine operations via --repository <path>.
2. In src/git/src/mcp_server_git/__init__.py, update the Click option help text for --repository to: 'Git repository path to confine operations to. Without this option, every reachable repository is in scope.'
3. In src/git/src/mcp_server_git/server.py, update validate_repo_path's docstring to document the None behavior.
4. In src/git/tests/test_entrypoints.py, update test_main_help to assert that the confinement message is present in --help output.

Behavior Before vs. After:

Verification Steps:

1. Ran python -m pytest tests/test_entrypoints.py -k test_main_help ➡️ PASSED
2. Verified mcp_server_git --help output renders formatted lines cleanly.
3. Verified test_main_without_repository_serves_none and test_python_dash_m_calls_main ➡️ PASSED.