Generate .circleci/config.yml from a repository scan

## Usage

`circleci config generate [path] [flags]`

## Arguments

`<path>` is optional and is the directory to scan. Defaults to
the current directory.

## Flags

Global flags: `-c, --config`, `--debug`, `--no-color`, `-q, --quiet` — see `circleci --help`.

## Examples

- Generate a config for the current directory: 
  `circleci config generate`
- Generate a config for a specific project path: 
  `circleci config generate ./my-app`
- Re-run is a no-op when a config already exists: 
  `circleci config generate`
- ✓ Using existing config at .circleci/config.yml

## Details

Detect the language stack, container image, and setup commands for a
repository, then write a starter pipeline to `<path>/.circleci/config.yml`.

If no supported stack is detected, a minimal cimg/base:stable template
with a placeholder build step is written instead so you have something
to iterate on.

If a config file already exists at that path, generate does not overwrite
it; it prints a confirmation and exits successfully. The .circleci/
directory is created if needed, and the file is written atomically.

