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/LaneGitHeader.tsx

@@ -36,7 +36,9 @@ const GIT_REFRESH_MS = 30_000;
 export function useLaneGitFacts(
   laneId: number,
   /** Read that session's own worktree instead (`?session=`). */
-  session?: string
+  session?: string,
+  /** Read that extra folder's repo instead (`?source=`). */
+  source?: number
 ): [LaneGitFacts | null, () => void] {
   const [facts, setFacts] = useState<LaneGitFacts | null>(null);
   // Bumped by the git action bar: a commit or a pull changes every fact on
@@ -54,7 +56,7 @@ export function useLaneGitFacts(
     let alive = true;
     const read = () => {
       api.lanes
-        .git(gitTarget(laneId, session))
+        .git(gitTarget(laneId, session, source))
         .then((f) => {
           if (alive) setFacts(keepIfSame(f));
         })
@@ -73,7 +75,7 @@ export function useLaneGitFacts(
       alive = false;
       clearInterval(timer);
     };
-  }, [laneId, session, nonce]);
+  }, [laneId, session, source, nonce]);
 
   return [facts, () => setNonce((n) => n + 1)];
 }
@@ -322,20 +324,23 @@ export function LaneGitCounts({
 export function LaneGitRailCounts({
   laneId,
   session,
+  source,
   onShowAll,
 }: {
   laneId: number;
   session?: string;
+  source?: number;
   /** Opens the Changes panel from the file list's "… more files". */
   onShowAll?: () => void;
 }) {
-  const [git] = useLaneGitFacts(laneId, session);
+  const [git] = useLaneGitFacts(laneId, session, source);
   return git?.available ? <LaneGitCounts git={git} vertical onShowAll={onShowAll} /> : null;
 }
 
 export function LaneGitHeader({
   laneId,
   session,
+  source,
   git,
   onChanged,
   onNewLane,
@@ -343,6 +348,8 @@ export function LaneGitHeader({
   laneId: number;
   /** Work in that session's own worktree (see `gitTarget`). */
   session?: string;
+  /** Work in that extra folder's repo instead (`lane_sources` id, see `gitTarget`). */
+  source?: number;
   git: LaneGitFacts | null;
   /** Called after a git bar action: the facts AND the panel's diff moved. */
   onChanged: () => void;
@@ -374,6 +381,7 @@ export function LaneGitHeader({
       <LaneGitBar
         laneId={laneId}
         session={session}
+        source={source}
         git={git}
         onChanged={onChanged}
         onNewLane={onNewLane}