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/lib/tmux.js

@@ -11,8 +11,9 @@
  * takes over an idle pane with `respawn-pane -- <argv>` rather than typing a
  * quoted line into that pane's shell, which keeps an API key passed in `env`
  * out of the pane's rendered text, its scrollback and the user's shell history.
- * `sendText` is the one primitive that does send keystrokes — a pane running an
- * interactive chat program that reads stdin as a message, not a shell.
+ * `sendText` is the one primitive that sends typed text — a pane running an
+ * interactive chat program that reads stdin as a message, not a shell — and
+ * `sendKey` presses one named key (Shift+Tab for `pane-mode.js`), never text.
  *
  * Server mode, one Linux user per account (S3): a member's panes live on THAT
  * member's tmux server, socket `<home>/.agenthub/tmux/sock` (`run-as.js`), so
@@ -547,6 +548,16 @@ function pasteText(name, text) {
   onServerOf(name, ["send-keys", "-t", pane(name), "Enter"]);
 }
 
+/** The pane's visible screen as plain text (no escape sequences). */
+function capturePane(name) {
+  return String(onServerOf(name, ["capture-pane", "-p", "-t", pane(name)]));
+}
+
+/** Press ONE named key (`BTab`, `Escape`) in the pane — a key name, never text. */
+function sendKey(name, key) {
+  onServerOf(name, ["send-keys", "-t", pane(name), key]);
+}
+
 /** Idempotent — a session that's already gone is not an error. */
 function killSession(name) {
   try {
@@ -657,6 +668,8 @@ module.exports = {
   sendCommand,
   sendText,
   pasteText,
+  capturePane,
+  sendKey,
   killSession,
   listSessions,
   liveSessions,