feat: v2.6.0 — Chat UI composer (+ menu, project files, activity panel, permission mode, Stop), git on a workspace's extra folders (#70), long paste as a file (#68), S3 server mode (experimental)

Chat UI:
- Each Claude/Codex session keeps its own chat/terminal view; Settings only
  sets how new sessions open.
- Composer + menu: upload, project files and folders (#54, also dragged from
  the Files/Changes tree), context files, recent uploads, skills, plugins.
- Activity side panel (progress, outputs, files read, uploads, memory,
  skills, connectors) with a file preview; edited-file counts per turn.
- Permission-mode chip (Shift+Tab steps, read back from the pane:
  server/lib/pane-mode.js, GET|POST /api/run/:id/permission-mode).
- Stop (Esc) while the main agent works; the status is checked per agent
  (with session_id /api/agents ignores its status filter) and re-read every
  5 s while Stop shows.
- Dropped images shown in the conversation (GET /api/run/drop/:name, the
  requester's own drop dir), full-size viewer.
- A plain-text paste over 5,000 characters is attached as pasted-text.txt,
  so no prompt reaches tmux/argv limits (#68).

Changes panel: every /api/lanes/:id/git* route takes ?source=<lane_sources
id> (sourceFolder; ENOSOURCE otherwise) and a folder picker switches the
whole panel to a workspace's extra folder (#70).

Server mode S3 (one Linux user per account) — experimental, verified with
fake sudo plus a P6 managed-settings spike on a test account:
- member tmux listing runs in the background (cached, 15 s => unknown);
- transcript snapshot copy is async, chunked, temp file + rename;
- Express Layer patch only on Express 4;
- tmux capturePane/sendKey go to the member's own server.

Docs: the multi-user design moved in as docs/plans/server-mode/README.md
(the AgentHub-Server repo is gone); S3-PLAN P6 result; LANES/API/openapi.

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

@@ -103,6 +103,8 @@ vi.mock("../../lib/api", async (importOriginal) => {
     api: {
       aiTools: { iconUrl: (id: string) => `/api/ai-tools/${id}/icon` },
       lanes: {
+        // The Changes panel's folder picker reads the workspace's extra folders.
+        sources: vi.fn().mockResolvedValue({ sources: [] }),
         list: vi.fn().mockImplementation(async () => {
           recordCall("GET", "/api/lanes");
           return { lanes: lanesToReturn, counts: countsToReturn };
@@ -434,15 +436,10 @@ describe("Workspace page — lane integration", () => {
 });
 
 describe("Workspace layout", () => {
-  it("opens the add-workspace modal from the empty banner when there is none", async () => {
-    const user = userEvent.setup();
+  it("shows no empty banner when there is no workspace (the sidebar's + adds one)", async () => {
     lanesToReturn = [];
     await renderWorkspace();
-
-    await user.click(screen.getByTestId("lane-empty"));
-    expect(
-      await screen.findByText("Create a workspace from a working directory")
-    ).toBeInTheDocument();
+    expect(screen.queryByTestId("lane-empty")).toBeNull();
   });
 
   it("shows the full card only for the selected lane, in the detail panel", async () => {