feat: resolve agent cards directly from ai-catalog references in --agent-card flag

## Which area does this relate to?
- [x] CLI code

## What kind of feature are you proposing?
New command or flag

## What would you like to see?

Allow `--agent-card` to accept an ai-catalog reference directly, so users can go from:

```
ai-catalog catalog add
ai-catalog pull -o agent-card.json
a2acli send --agent-card agent-card.json
```

to:

```
ai-catalog catalog add
a2acli send --agent-card
```

The CLI should detect when the value passed to `--agent-card` is an ai-catalog reference rather than a local file path, resolve it via the registered catalogs maintained by the ai-catalog CLI, fetch the Agent Card JSON, and use it as if it had been supplied as a local file.

To also support one-shot resolution without a pre-registered catalog, a scoped format should be accepted:

```
a2acli send --agent-card "@"
```

This mirrors the pattern `ai-catalog pull --scope ` but inline as a single flag value.

### Resolution order

1. If the value is a valid local file path that exists — use it as today (no change).
2. If the value matches `@` — resolve `` against `` (URL or registered catalog name) via the ai-catalog CLI.
3. Otherwise treat the value as a bare `` and resolve it against all registered ai-catalog catalogs (equivalent to `ai-catalog pull `).

### Dependency

Requires the [ai-catalog CLI](https://github.com/Agent-Card/ai-catalog-cli) to be installed and accessible on `$PATH`. If it is not found and a catalog reference is supplied, the CLI should emit a clear error pointing to the install instructions.

## Why is this important?

Today, using a catalog-hosted agent requires a manual two-step dance: pull the card to a temp file, then pass that file. This is friction for humans and makes scripting / automation more fragile (temp file lifecycle, naming, cleanup). Native reference resolution makes catalog-backed agents first-class citizens alongside local card files.

## Who benefits from this feature?

- Developers scripting agent calls in CI/CD pipelines who want a single stable reference rather than managing ephemeral JSON files.
- AI coding agents driving the CLI (e.g. Claude Code) that can use a catalog reference directly without filesystem side-effects.
- Any user who has already invested in organising agents in an ai-catalog and wants a seamless `a2acli` experience.

## Would you be interested in helping build this feature?
Yes — I'd like to implement it

## Additional context

- ai-catalog CLI repo: https://github.com/Agent-Card/ai-catalog-cli
- Relevant ai-catalog commands: `catalog add`, `pull`, `pull --scope`
- The `@` separator in `@` is chosen to match common toolchain conventions (npm scopes, OCI references); open to alternatives if there is a conflict.

## Code of Conduct
- [x] I agree to follow this project's Code of Conduct