feat(ui): AI Tools modal, one-button lane tool picker, AgentHub settings in Overview, markdown preview

- AI Tools moves from a Settings section into a modal behind a button at the
  Settings tab bar's right edge (AiToolsModal, `actions` on PageTabs,
  /settings#ai-tools); rows get an enable switch, Default badge, and an
  Update button for every tool whose catalog names an updater (no version
  lookup on open); not-installed tools fold behind a disclosure, a finished
  job's log folds to one line; shared AiToolIcon
- codex updates through its own `codex update`, openclaw/hermes gain
  `update --yes`; update-all runs every installed updater and skips an npm
  one only when npm does not manage the binary
- Add-lane tool picker is one button showing the chosen tool that opens the
  enabled tools as buttons
- Claude Code / Codex AgentHub sub-tab folded into Overview (home editor,
  hooks, pricing); the path list drops CLAUDE_HOME/CODEX_HOME;
  HomeDirSetting shows the saved value only while the field differs
- Preview/Raw toggle for markdown files in the config FileViewer and the
  Changes panel's whole-file view (MarkdownPreview: react-markdown +
  remark-gfm, raw HTML never run, frontmatter as a code block)
- docs and the CLAUDE.md marker updated

client/src/pages/__tests__/CodexConfig.test.tsx

@@ -123,7 +123,7 @@ afterEach(() => {
 });
 
 describe("CodexConfig", () => {
-  it("edits CODEX_HOME on the AgentHub sub-tab and reports lanes whose router layer failed", async () => {
+  it("edits CODEX_HOME on the Overview and reports lanes whose router layer failed", async () => {
     stubCodexApi({});
     codexHomeMock.get.mockResolvedValue({ codex_home: "/home/.codex", env_owned: false });
     codexHomeMock.set.mockResolvedValue({
@@ -136,7 +136,8 @@ describe("CodexConfig", () => {
     await settle();
     const overviewCalls = codexConfigMock.overview.mock.calls.length;
 
-    await userEvent.click(screen.getByRole("tab", { name: /agenthub/i }));
+    // No AgentHub sub-tab: its cards sit under the Overview, the page's first tab.
+    expect(screen.queryByRole("tab", { name: /agenthub/i })).toBeNull();
     const input = await screen.findByDisplayValue("/home/.codex");
     expect(await screen.findByText("Codex hooks")).toBeInTheDocument();
     expect(screen.queryByPlaceholderText(/search/i)).not.toBeInTheDocument();