#8306 Revert "Settings > Browser > Extensions: import, add-unpacked, and live enable/disable for web extensions"

Reverts manaflow-ai/cmux#7699

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Summary by cubic

Reverts the Browser Extensions feature and all WebKit web‑extension hosting. Removes the browser.webExtensions setting, related UI, discovery/hosting code, and dependent APIs to restore the previous browser behavior.

• Refactors

  • Removed extension settings and UI: browser.webExtensions, BrowserWebExtensionsCard, discovery/import flows, and all related localizations.
  • Deleted web‑extension runtime: BrowserWebExtensionSupport and all popout/permission/toolbar/tab adapter types.
  • Simplified browser code paths: moved external navigation helpers into BrowserPanel; removed extension navigation/context switching.
  • Removed seams from AppDelegate, DockSplitStore, TabManager, and Workspace for extension ownership, routing, and focus.
  • JSONConfigStore: removed update(_:); simplified JSONConfigStoreReadError.
  • JSONValueModel: removed first‑value/write tracking state; kept a simpler observe/set/reset flow.
  • BrowserPrewarmedWebViewPool and upgrade prewarmers: dropped browserWebExtensionHost parameters; updated call sites.
  • Pruned tests and strings tied to extensions; updated navigation and popup tests accordingly.

• Migration

  • The browser.webExtensions key is no longer read; previously configured extensions will not load.
  • Remove uses of SettingsHostActions.discoverBrowserWebExtensions(), .browserWebExtensionsSupported(), and .browserWebExtensionLoadErrorUpdates().
  • Replace JSONConfigStore.update(_:) calls with explicit read/modify/set within the store actor.
  • Drop browserWebExtensionHost parameters and references; use BrowserPrewarmedWebViewPool.prewarm(url:profileID:) and claim(url:profileID:websiteDataStore:).

Summary by CodeRabbit

• Changes
  • Removed built-in browser Web Extension discovery, loading, permissions, toolbar actions, popouts, and related settings.
  • Simplified browser navigation and external-link handling, including support for HTTP(S) fallbacks from intent links.
  • Improved browser popup data-store handling and streamlined web view lifecycle behavior.
  • Terminal configuration input now waits for surfaces to become ready before being delivered.
  • Simplified settings updates and removed obsolete browser extension configuration options.