docs(readme): remove preview notice

[moonyue-w]

Summary

Remove the preview/pre-1.0 warning callout from the TypeScript SDK README. The npm badge now leads directly into the SDK introduction.

Why

Remove the prominent preview notice as requested. This is a documentation edit; it does not change the SDK version, APIs or compatibility policy.

Verification

• npm run build
• npm run typecheck
• npm test
• npm run docs:check
• Relevant examples run or import successfully — all 32 README TypeScript snippets compile in docs-check
• Account-backed integration tests run, or not required — not required for this text-only deletion

npm run docs:check and git diff --check passed. Standalone build, typecheck and runtime tests were not repeated locally for this README-only change.

Impact

• Public API or behavior changed
• User-facing changes added to CHANGELOG.md under Unreleased, or not applicable (explain below)
• CommonJS, ESM, and subpath exports remain valid — unchanged
• Generated API documentation updated
• Contract or wire-field fixtures updated
• Go and Python parity considered — this request concerns only the TypeScript README
• Migration notes added for a breaking change
• No credentials, .env.live, logs, or generated test output committed

No runtime change, generated reference change, fixture change or migration is involved. A release changelog entry is not needed for removing this README notice.