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/lane-sources.js

@@ -106,6 +106,28 @@ function addFolder(laneId, input, { allowed = () => true } = {}) {
   return insertSource(laneId, "folder", resolved, path.basename(resolved) || resolved, null);
 }
 
+/**
+ * An extra folder's directory for the git routes (`?source=<id>`): only a
+ * `folder` row of THIS workspace that is still a directory — the request names
+ * a row id, never a path.
+ * @throws ENOSOURCE otherwise.
+ */
+function sourceFolder(laneId, sourceId) {
+  const row = /^\d+$/.test(String(sourceId))
+    ? db
+        .prepare("SELECT path FROM lane_sources WHERE id = ? AND lane_id = ? AND kind = 'folder'")
+        .get(Number(sourceId), laneId)
+    : null;
+  let dir = null;
+  try {
+    dir = row && fs.statSync(row.path).isDirectory() ? fs.realpathSync(row.path) : null;
+  } catch {
+    /* gone since it was added */
+  }
+  if (!dir) throw fail("ENOSOURCE", "that folder is not one of this workspace's folders");
+  return dir;
+}
+
 /** A file name AgentHub can store: no directory part, no control or path characters. */
 function safeName(raw) {
   const base = path.basename(String(raw || "")).replace(/[^\p{L}\p{N}._ -]/gu, "_");
@@ -338,6 +360,7 @@ module.exports = {
   contextDir,
   listSources,
   addFolder,
+  sourceFolder,
   addFile,
   removeSource,
   removeLaneContext,