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/components/lanes/__tests__/LaneChangesPanel.test.tsx

@@ -255,6 +255,36 @@ describe("LaneChangesPanel", () => {
     expect(view).toHaveTextContent("keep");
   });
 
+  it("opens a file asked for from outside (a chat's Edited files card), once per request", async () => {
+    const view = render(
+      <LaneChangesPanel laneId={7} openRequest={{ path: "server/lib/git-ops.js", nonce: 1 }} />
+    );
+    await waitFor(() =>
+      expect(api.lanes.changes).toHaveBeenCalledWith(7, {
+        range: "work",
+        path: "server/lib/git-ops.js",
+        context: 3,
+      })
+    );
+    vi.mocked(api.lanes.changes).mockClear();
+    // The same request again changes nothing; a new nonce opens the next one.
+    view.rerender(
+      <LaneChangesPanel laneId={7} openRequest={{ path: "server/lib/git-ops.js", nonce: 1 }} />
+    );
+    view.rerender(<LaneChangesPanel laneId={7} openRequest={{ path: "src/main.ts", nonce: 2 }} />);
+    await waitFor(() =>
+      expect(api.lanes.changes).toHaveBeenCalledWith(7, {
+        range: "work",
+        path: "src/main.ts",
+        context: 3,
+      })
+    );
+    expect(api.lanes.changes).not.toHaveBeenCalledWith(
+      7,
+      expect.objectContaining({ path: "server/lib/git-ops.js" })
+    );
+  });
+
   it("asks for every file's hunks in review mode", async () => {
     const user = userEvent.setup();
     render(<LaneChangesPanel laneId={7} />);