[Feature] Expand azd ai agent sample discovery

## Problem

`azd ai agent sample list` currently exposes only a limited set of samples. This can make it difficult for users to discover examples that match their scenario and understand the range of agent patterns supported by azd.

## Proposal

Discuss whether azd should:

- Add more samples covering common agent scenarios and Azure services.
- Introduce a curated or featured subset of samples that highlights recommended, high-quality, or newly updated examples.
- Add metadata such as categories, tags, maturity, or supported scenarios to improve discovery.
- Define contribution and maintenance criteria so the list remains useful and up to date.

## Questions for discussion

- Which scenarios should be prioritized for additional samples?
- Should `sample list` show every available sample, or distinguish featured samples from the broader catalog?
- How should featured samples be selected and refreshed?
- Would categories, tags, or filters provide enough discoverability without making the command harder to use?

Feedback from maintainers and users is welcome before deciding on the scope and implementation approach.

Start by reviewing the current `azd ai agent sample list` command and how its available samples are sourced. The issue does not name files or tests, and completion criteria are not defined; first establish the prioritized scenarios, catalog or featured-list behavior, metadata needs, and maintenance rules before implementation.

Written by the indexing model from the issue text.