Add an init command to create agents-cli-manifest.yaml for existing ADK projects

### What is your feature suggestion?

I would like agents-cli to provide a lightweight initialization command that writes `agents-cli-manifest.yaml` into an existing ADK project, without scaffolding a new project or adding deployment files.

For example, the command could be:

agents-cli init

or:

agents-cli scaffold init

Ideally this would feel similar to `hatch new --init`: when run from an existing project root, it detects reasonable defaults and creates the minimal project metadata needed by agents-cli commands.

A possible generated manifest could be:

name: my-agent
agent_directory: app
region: us-east1
create_params:
deployment_target: none

It would also be helpful to support flags such as:

agents-cli init --agent-directory app --name my-agent
agents-cli init --dry-run
agents-cli init --yes

The command should avoid overwriting an existing `agents-cli-manifest.yaml` unless an explicit `--force` flag is provided.

### What will this enable you to do?

This would make it much easier to adopt agents-cli in existing ADK projects.

Right now, after adding eval files to an existing project by skill equiped coding agent, running:

agents-cli eval run --all

fails with:

Error: No agents-cli-manifest.yaml found in the current directory or its parents.
Run this command from your project root, or create a project first:
agents-cli create my-agent

For an existing ADK project, I do not necessarily want to create a new project or run a broader scaffold/enhance flow. I only need the minimal manifest so commands like `agents-cli eval run`, `agents-cli info`, and other project-aware commands can locate the project root and agent directory.

A lightweight `init` command would provide a clear migration path:

1. Start with an existing ADK project.
2. Add evalsets/config.
3. Run `agents-cli init`.
4. Run `agents-cli eval run --all`.

This would reduce friction for users who already have ADK agents and want to gradually adopt agents-cli workflows.

### Additional context

The ideal UX is similar to `hatch new --init`, where an existing project can be initialized in place.
https://hatch.pypa.io/dev/cli/reference/#hatch-new

Nice-to-have detection behavior:

- infer `name` from `pyproject.toml` or the current directory name
- infer `agent_directory` from common ADK layouts such as `app/`, a package listed in `pyproject.toml`, or a directory containing `agent.py` / `root_agent.yaml`
- default `deployment_target` to `none` so local eval usage works without opting into deployment scaffolding
- print the generated manifest and explain what was detected
- provide flags for explicit values when auto-detection is wrong

This would complement `agents-cli create` for new projects and `agents-cli scaffold enhance` for adding broader scaffold/deployment support, while covering the smaller "initialize this existing ADK project for agents-cli" use case.