docs: polish repo for public beta

[cesarenaldi]

Summary

• add structured GitHub issue forms for bug reports, feature requests, and general feedback/questions
• add beta-era contribution and security guidance
• refresh README installation and beta positioning
• move CLAUDE.md to the repository root
• remove stale wording from the SDK direction doc

Verification

• make lint
• make format-check
• make typecheck
• make test

Follow-up

• Re-check GitHub private vulnerability reporting once the repository is public.

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Note

Low Risk
Low risk because changes are documentation and GitHub metadata only, with no runtime or API behavior changes.

Overview
Prepares the repo for a public beta by adding structured GitHub issue forms (bug, feature, and general feedback) and disabling blank issues to standardize triage.

Updates project docs to emphasize beta status, clarify that external PRs aren’t broadly accepted yet, add a SECURITY.md policy pointing reporters to private vulnerability reporting, and refresh wording in README.md (including prerelease install commands) and docs/sdk-direction.md.

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