refactor(mcp): atomic calls via path; remove set_repo

[mars167]

Summary

• Remove stateful MCP tool set_repo to keep MCP tool calls atomic.
• Add optional path parameter to get_repo so every MCP tool supports repository scoping.
• Update docs and tests accordingly.

Compatibility

• CLI behavior unchanged.
• MCP callers should pass path per tool call (recommended), or start the server with git-ai ai serve --path <repo>.

Test

• npm test