feat: add Sinhala (si) README translation

[Pranav-0440]

Summary

Adds a complete Sinhala (si) translation of the main README to improve accessibility and localization coverage for Sinhala-speaking developers.

Changes

• Added readme/in-si.md with a full Sinhala translation of the README
• Ensured formatting matches existing in-*.md localized files
• Added changeset to follow Lingo.dev release requirements

Testing

Business logic tests added:
(Not applicable — documentation-only PR)

• No new business logic introduced
• No runtime behavior modified
• All existing tests pass locally (pnpm turbo test)

Checklist

• Changeset added
• No breaking changes
• Follows updated folder structure (readme/in-si.md)
• CI checks passing

Closes #747

[Pranav-0440]

Hi maintainers 👋,
The CI is failing due to insufficient GITHUB_TOKEN permissions on forked PRs:

Actor does not have write permissions to the repository

This appears to be a known limitation of the “prepare” step for contributors without write access.

The documentation-only changes and changeset are complete, and this PR is ready for review once a maintainer reruns or bypasses the workflow.
Thank you!