Remove Phoenix & Notebooks - Phase 2: Remove in-app notebook authoring & rendering (#2515)

Delete the nteract rendering engine, notebook tabs, panes, the read-only viewer, Schema Analyzer (pulled forward from Phase 3), and all UI entry points that open notebooks. Decouple surviving files (Explorer, NotebookManager, useNotebook, ResourceTreeAdapter) with minimal edits, keeping GitHub/Juno/Phoenix wiring for later phases.

Removed 22 zero-importer notebook-only npm deps; re-added phantom transitively-hoisted deps still used by surviving code: xterm, xterm-addon-fit (CloudShell), d3-collection (Graph), @nteract/myths (@nteract/core).

Verified: compile, compile:strict, lint (0 errors), format:check, test (1945 passing), build:ci all green.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

jest.config.js

@@ -70,7 +70,6 @@ module.exports = {
   moduleNameMapper: {
     "^.*[.](png|gif|less|css)$": "<rootDir>/mockModule",
     "(.*)$[.](svg)": "<rootDir>/mockModule/$1",
-    "@nteract/stateful-components/(.*)$": "<rootDir>/mockModule",
     "@fluentui/react/lib/(.*)$": "@fluentui/react/lib-commonjs/$1", // https://github.com/microsoft/fluentui/wiki/Version-8-release-notes
     "monaco-editor/(.*)$": "<rootDir>/__mocks__/monaco-editor",
     "^dnd-core$": "dnd-core/dist/cjs",