[ZEPPELIN-6670] Add Shared Notebook Core mount contract

[voidmatcha]

What is this PR for?

Add @zeppelin/notebook-core as a shared package for the Angular host and React remote.

Define common types for the read-only snapshot, getSnapshot, subscribe, and mount props. Add dependency-boundary checks and connect the package build, typecheck, and unit tests to the existing Maven/CI lifecycle.

This change does not implement the Core runtime. Separate-build object identity and notebook route integration will be verified in ZEPPELIN-6674 and ZEPPELIN-6675.

What type of PR is it?

Improvement

What is the Jira issue?

ZEPPELIN-6670

How should this be tested?

The following checks passed locally in zeppelin-web-angular:

• npm run test:notebook-core: 103 tests passed
• npm run typecheck:notebook-core: passed
• npm run test:shell: 49 tests passed
• npm run check:notebook-parity-scenarios: 21 tests and the registry check passed
• ESLint and Prettier checks for the changed code: passed

Screenshots (if appropriate)

No UI changes

Questions:

• Does the license files need to update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? Updated README and AGENTS.md with the check commands and file locations.