Record hosted agent deployment mode telemetry

## Goal

Record the resolved hosted-agent deployment path in azd telemetry.

## Rationale

Hosted agents can upload code, build a container image, or use a user-provided prebuilt image. `project.service.languages` describes the manifest language but does not identify which deployment path actually ran, which limits adoption and troubleshooting insights.

## Scope

- Define `agent.deploy.mode` as a `SystemMetadata` field for `FeatureInsight`.
- Emit only the fixed values `code`, `container`, and `byo_image`.
- Have the extension label its existing package or publish artifacts after resolving the lifecycle path.
- Have azd core validate the value and record it on its active recording span.
- Document the field and add focused coverage for each mode.

## Acceptance criteria

- The three supported paths record the corresponding fixed enum.
- User-provided image references, registry names, repositories, and tags are never emitted.
- The extension remains independently buildable against its released azd SDK dependency.
- The field, schema, and telemetry matrix remain aligned.