chore: add deprecated comment to FeaturesRepository

[eunjae-lee]

What does this PR do?

Adds a @deprecated JSDoc annotation to the FeaturesRepository class in packages/features/flags/features.repository.ts.

This will cause IDEs to show deprecation warnings when the class is used, signaling to developers that this class should not be used for new code.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - documentation-only change.
• I confirm automated tests are in place that prove my fix is effective or that my feature works. N/A - no functional changes.

How should this be tested?

No testing required - this is a documentation-only change that adds a @deprecated JSDoc tag.

To verify:

1. Open packages/features/flags/features.repository.ts in an IDE
2. Confirm the @deprecated tag is present in the class JSDoc
3. Import/use FeaturesRepository elsewhere and verify the IDE shows a deprecation warning

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Requested by: @eunjae-lee
Link to Devin run: https://app.devin.ai/sessions/7628fd1fea3c433cba5bd8271a5f7911

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