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.

server/__tests__/server-mode-workspaces.test.js

@@ -143,6 +143,12 @@ describe("server mode", () => {
     assert.equal((await get("/api/lanes", "alice")).counts.total, 1);
     assert.deepEqual(await laneIds("boss"), [LANE_A, LANE_B]);
     assert.deepEqual(await laneIds("ana"), [LANE_A, LANE_B]);
+    // Each carries its owner's name, for the staff sidebar's per-user groups.
+    const owners = (await get("/api/lanes", "ana")).lanes.map((l) => [l.id, l.owner_name]);
+    assert.deepEqual(owners.sort(), [
+      [LANE_A, "alice"],
+      [LANE_B, "bob"],
+    ]);
     assert.deepEqual(
       (await get("/api/workspaces", "bob")).lanes.map((l) => l.id),
       [LANE_B]