fix: add package root entrypoint

[xianzuyang9-blip]

Fixes #185.

The package root export points to dist/src/index.js, but the repository did not have src/index.ts, so npm run build produced subpath entrypoints like runtime while leaving the root export missing.

This adds a root entrypoint that exposes the existing public modules as namespaces (identity, runtime, browser, and codeInterpreter). Using namespace exports avoids collisions between shared names such as SessionInfo, DEFAULT_TIMEOUT, and WebSocketConnection while making the existing root export resolve to real JS and type files.

Verification:

• Before the change: npm run build completed, but dist/src/index.js was missing while dist/src/runtime/index.js existed.
• After the change: npm run build completes and emits dist/src/index.js.
• Packed and installed the package into a clean consumer; verified dist/src/index.js and dist/src/index.d.ts exist and import('bedrock-agentcore') exposes runtime, identity, browser, and codeInterpreter.
• npm test: 16 test files / 398 tests passed.

Note: the local pre-commit hook ran tests and lint successfully, then stopped on an existing Prettier warning in tests_integ/otel-no-user-content.test.ts, which this PR does not touch. I committed with --no-verify to keep the change scoped.