GitHub Action: azure/setup-cosmosdb-shell for CI workflows

## Feedback source
External user feedback (travel multi-agent workshop team).

## Problem
There is no official GitHub Action to install/setup CosmosDB Shell in CI workflows. Users must manually download, extract, and add to PATH — adding boilerplate and fragility to every workflow.

## Proposed solution
Publish an official **azure/setup-cosmosdb-shell** GitHub Action that:
- Accepts a version input (with a sensible default/latest)
- Downloads the correct platform-specific package
- Extracts and adds to PATH
- Caches the binary across runs (using actions/cache or tool-cache)
- Verifies checksum/signature

## Example usage
```yaml
steps:
- uses: azure/setup-cosmosdb-shell@v1
with:
version: 1.0.213-preview
```n
## Acceptance criteria
- [ ] Action published to GitHub Marketplace
- [ ] Supports Linux, macOS, Windows runners
- [ ] Built-in caching (tool-cache pattern)
- [ ] Version pinning and 'latest' support
- [ ] Documented in CI/CD guide

Start from the example workflow in the issue and review the CI/CD guide to understand the expected user-facing setup. Done means a published azure/setup-cosmosdb-shell action supports the listed platforms, version modes, caching, verification, and documented CI/CD usage.

Written by the indexing model from the issue text.