examples-migrate tool

Part of #4831.

RFC: https://github.com/Azure/azure-rest-api-specs/blob/rfc/unified-examples-format/documentation/rfc/unified-examples-format.md

Build **`examples-migrate`**: convert existing `x-ms-examples` JSON into `examples.yaml`.

### Scope
- Read all `x-ms-examples` JSON for a service and emit a single `examples.yaml` (or `examples/.yaml` for large services).
- **Dedup across versions** into `since` variants (full restatement, no partial overrides).
- **Normalize embedded version strings to `{api-version}`** in headers and body values (e.g. `Location`, `Azure-AsyncOperation`, `nextLink`) so trivially-versioned variants collapse.
- Emit interface-relative operation keys under a single `$namespace`.

Depends on #4832 (schema).

Start with the unified examples format RFC at documentation/rfc/unified-examples-format.md and the schema work in #4832. Inspect the existing x-ms-examples JSON for a service and define the migration entry point from the requested output structure. Done means producing examples.yaml or interface files with deduplicated since variants, normalized {api-version} values, and interface-relative operation keys under one $namespace.

Written by the indexing model from the issue text.