Remove pipeline variables section from azure.yaml per azd 1.17.0+

[Copilot]

As of Azure Developer CLI version 1.17.0, the pipeline: variables: section in azure.yaml is no longer needed. The Azure Developer CLI now auto-detects required variables from main.parameters.json.

This PR updates the repository's azure.yaml by:

1. Removing the entire pipeline: block (23 lines) including all nested variables: entries
2. Updating the azd version requirement from ">= 1.10.0" to ">= 1.17.0" to enforce the modern azd version
3. Bumping the template version from 0.0.1 to 0.0.2 to indicate the change

Benefits:

• Simplifies configuration (variables now inferred automatically)
• Reduces duplication and risk of drift between pipeline variables and parameter definitions
• Aligns with current azd best practices

Validation:

• Confirmed that 18 out of 20 pipeline variables are auto-detected from infra/main.parameters.json
• Verified YAML structure remains valid
• Tested that core application modules continue to work correctly

The two variables not found in main.parameters.json (AZURE_OPENAI_VERSION and AZURE_OPENAI_EMBEDDING_COLUMN) appear to be legacy/unused, confirming that the auto-detection approach is sufficient.

Fixes #261.

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