Remove Phoenix & Notebooks - Phase 4: Remove GitHub integration (#2528)

Removes the GitHub notebook-repo integration that only existed to pin/browse notebook repositories. Deletes src/GitHub/, the GitHub controls/panes, GitHubUtils, JunoUtils, and the connectToGitHub webpack entry. Decouples NotebookManager, useNotebook, the resource tree, and Explorer from GitHub wiring. Trims JunoClient's GitHub-only methods while keeping the Schema and gallery methods. Removes GitHub config fields from ConfigContext and strips the dead github:// branches from NotebookUtil.

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

webpack.config.js

@@ -110,7 +110,6 @@ module.exports = function (_env = {}, argv = {}) {
     quickstart: "./src/quickstart.ts",
     hostedExplorer: "./src/HostedExplorer.tsx",
     selfServe: "./src/SelfServe/SelfServe.tsx",
-    connectToGitHub: "./src/GitHub/GitHubConnector.ts",
     redirectBridge: "./src/redirectBridge.ts",
     ...(mode !== "production" && { testExplorer: "./test/testExplorer/TestExplorer.ts" }),
     ...(mode !== "production" && {
@@ -139,11 +138,6 @@ module.exports = function (_env = {}, argv = {}) {
       template: "src/hostedExplorer.html",
       chunks: ["hostedExplorer"],
     }),
-    new HtmlWebpackPlugin({
-      filename: "connectToGitHub.html",
-      template: "src/connectToGitHub.html",
-      chunks: ["connectToGitHub"],
-    }),
     new HtmlWebpackPlugin({
       filename: "selfServe.html",
       template: "src/SelfServe/selfServe.html",