fix: add root package entrypoint

[hsduren]

Summary

• add the missing root SDK entrypoint at src/index.ts
• export core classes/functions directly and expose module namespaces for runtime, identity, browser, and code interpreter APIs
• make the package manifest's existing main, module, types, and root exports["."] targets resolvable after build

Validation

I verified the published package currently declares root entrypoints that are not present in the tarball:

curl -L --max-time 30 -sS -o pkg.tgz https://registry.npmjs.org/bedrock-agentcore/-/bedrock-agentcore-0.2.4.tgz
tar -xzf pkg.tgz
jq '{main,module,types,exports:.exports["."]}' package/package.json
for f in dist/src/index.js dist/src/index.d.ts; do test -e "package/$f"; echo "$f rc=$?"; done

Result before this change: both dist/src/index.js and dist/src/index.d.ts are missing.

Local validation after this change:

npm ci --registry=https://registry.npmjs.org --strict-ssl=false --ignore-scripts --no-audit --no-fund
npm run build
test -f dist/src/index.js
test -f dist/src/index.d.ts
node -e "import('./dist/src/index.js').then(m=>console.log(Object.keys(m).filter(k=>/BedrockAgentCoreApp|RuntimeClient|Browser|CodeInterpreter|withAccessToken|runtime|identity|browser|codeInterpreter/.test(k)).sort()))"
git diff --check

npm run build passes, the root JS and declaration files are generated, and the root entrypoint imports successfully.

AI assistance disclosure: I used Codex to identify the missing package root entrypoint, prepare the patch, and run local validation. I reviewed the final diff before submission.