feat(workspace): AI tool per session, one place to start a session, ended sessions open like Claude Desktop

- A session picks its own AI tool: `tool` on POST /api/lanes/:id/start
  (a catalog id, known + enabled, else 400 EBADTOOL/EDISABLED); a resume
  keeps the tool its session ran on (`sessions.tool`). The workspace's
  `lanes.tool` is now only the default.
- The workspace home's prompt box is the one place a session starts. Its
  fields run in order: AI → AI Router (Native or a profile) → Model (the
  tool's own list, or the router's models) → Permission (Claude mode /
  Codex sandbox+approval / catalog bypass) → Effort.
- The pane's run form (RunSetup) is gone: New run, a single pane's exit and
  an empty split pane's New session all go to the home. RunSetup.tsx keeps
  PromptEditor, CwdAutocomplete and ModelPicker (new `choices` prop).
- The tool icon moved from the sidebar's workspace row to each session row
  (right edge, `session.tool`); the split-pane header shows the run's tool.
- An ended session opens in its workspace (`&session=`, SessionReplay.tsx):
  its conversation read-only plus a prompt box; only a SENT message resumes
  it (resumeSessionId + prompt, first free slot). The console is only
  hidden behind it.
- Analyst button beside Files/Changes in the right panel opens
  /sessions/:id for the opened session.
- Gemini CLI ships its own icon (client/public/ai-tools/gemini.svg, from
  zonalogo.com).
- RouterPicker always shows (Native disabled for tools without router
  support); useAiRouters() exported.
- Tests: server start-with-tool cases; SessionReplay, home field order/tool/
  router models, empty pane, Analyst; dropped the cwd-field suites
  (Run.defaultCwd, Workspace.laneCwd). Docs: CLAUDE.md rules, API.md,
  LANES.md, README, ARCHITECTURE, INSTALL.

client/src/components/__tests__/WorkspaceTree.test.tsx

@@ -230,7 +230,7 @@ describe("WorkspaceTree", () => {
     expect(screen.getByTestId("workspace-session-dot-s-bad")).toHaveClass("bg-status-danger");
   });
 
-  it("opens a live session's pane and an ended session's transcript, resuming nothing", async () => {
+  it("opens a live session's pane and an ended session's conversation, resuming nothing", async () => {
     const user = userEvent.setup();
     renderTree();
     await user.click(await screen.findByTestId("workspace-toggle-1"));
@@ -244,7 +244,7 @@ describe("WorkspaceTree", () => {
       "/workspace?lane=1&run=agenthub-lane-1-2&pane=1"
     );
     await user.click(screen.getByTestId("workspace-session-s-old"));
-    expect(screen.getByTestId("where").textContent).toBe("/sessions/s-old");
+    expect(screen.getByTestId("where").textContent).toBe("/workspace?lane=1&session=s-old");
     expect(api.lanes.action).not.toHaveBeenCalled();
   });