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/routes/lanes.js

@@ -15,6 +15,7 @@ const path = require("node:path");
 const { db } = require("../db");
 const lanesLib = require("../lib/lanes");
 const { sessionWorktree, removeSessionWorktree } = require("../lib/session-worktree");
+const { sourceFolder } = require("../lib/lane-sources");
 const { suggestCommitMessage } = require("../lib/commit-message");
 const aiTools = require("../lib/ai-tools");
 const {
@@ -951,8 +952,9 @@ router.get("/:id/git", async (req, res) => {
       available: true,
       ...(await gitFacts(t.cwd)),
       // What the Changes panel's "vs base" compares against: the lane's own
-      // base, else the repo's default branch.
-      base: t.lane.base_branch || (await defaultBranch(t.cwd)),
+      // base, else the repo's default branch (an extra folder is another
+      // repo: always its own default branch).
+      base: (!t.source && t.lane.base_branch) || (await defaultBranch(t.cwd)),
       ...(t.worktree ? { worktree: t.worktree } : {}),
     });
   } catch {
@@ -1702,13 +1704,24 @@ function gitErrorCode(err) {
  * Which tree a git route works in: the workspace's own folder, or — with
  * `?session=<id>` — that session's worktree (`claude --worktree`), resolved by
  * `sessionWorktree` from the session ROW (owned by this workspace, registered
- * by `git worktree list`), never from a path in the request. An unresolvable
- * session is a 400, not a silent fall back to the main folder: a commit or a
- * discard must never land in the wrong tree.
+ * by `git worktree list`), or — with `?source=<id>` — one of the workspace's
+ * extra folders, resolved by `sourceFolder` from its `lane_sources` ROW; never
+ * from a path in the request. An unresolvable session (400) or source (404,
+ * like another workspace's row) is never a silent fall back to the main
+ * folder: a commit or a discard must never land in the wrong tree.
  */
 async function gitTargetOr404(req, res) {
   const lane = laneOr404(req, res);
   if (!lane) return null;
+  if (req.query.source !== undefined) {
+    try {
+      const cwd = sourceFolder(lane.id, req.query.source);
+      return { lane, cwd, worktree: null, source: true };
+    } catch (err) {
+      res.status(404).json({ error: { code: err.code, message: err.message } });
+      return null;
+    }
+  }
   const session = req.query.session;
   if (session === undefined) return { lane, cwd: lane.cwd, worktree: null };
   try {