feat(server-s3): one Linux user per account (S3) — experimental

Server mode can now run each account as its own Linux user (ah-<name>)
when `agenthub server install-system` has set S3 up. The dashboard
reaches a member only through sudo -u / write-as / pane-launch, reads a
member's files only through owner- and type-checked opens, and hooks
are authenticated by peer uid. Local mode and server mode without S3
are unchanged.

EXPERIMENTAL: tested with fake sudo and fake accounts only. It has not
yet been verified on a real install with real member accounts, sudo or
a real Codex login. Do not offer it to real users yet.

- S3 core: provisioning, per-member tmux servers, the proxy key
  gateway, shared tools in /opt, dropped capabilities for children
  (P0–P5)
- P4: plan and quota per member home
- P6: ~/.claude.json seed; opt-in plugin baseline installed as the
  member; managed-settings spike script (not run)
- P7: Codex account pool (`agenthub accounts add --tool codex`)
- Roles merged with main's admin / analyst / developer: analysts write
  only their own rows, under S3 as well
- Per-account prefs: one implementation (Chat UI + onboarding)
- Security reviews F1–F49 (five rounds), each fix with a regression
  test: symlink/TOCTOU-safe member reads, no host writes in a member
  home, credentials and the data dir never served, owner and folder
  account must agree, pane tokens bound to the pane's uid, bounded
  sudo/tmux/git calls and member reads (no hang or crash from a member),
  async route rejections answer 500 instead of exiting
- Docs: docs/SERVER-MODE.md (S3 model, accepted ceilings, operator
  runbook), docs/plans/server-mode/S3-PLAN.md, CLAUDE.md S3 rules

Tests: server 2838/2841 (3 need 127.0.0.1:3000, held on this box);
client 1162/1162; tsc clean; headers check passes.

Co-Authored-By: Claude <noreply@anthropic.com>

client/src/components/run/RunSetup.tsx

@@ -163,6 +163,8 @@ interface PromptEditorProps {
   /** An image upload started (true) or settled (false): the caller holds its
    *  send button meanwhile, or the prompt would go out without the image. */
   onBusyChange?: (busy: boolean) => void;
+  /** The workspace (id, or a pane's run id) whose panes read the files. */
+  lane?: number | string;
 }
 
 /** An attached file: its path on the server's machine, and a local preview
@@ -187,6 +189,7 @@ export function PromptEditor({
   onImagesChange,
   bare,
   onBusyChange,
+  lane,
 }: PromptEditorProps) {
   const { t } = useTranslation("run");
   const taRef = useRef<HTMLTextAreaElement | null>(null);
@@ -325,7 +328,7 @@ export function PromptEditor({
     const picked = Array.from(list ?? []);
     if (picked.length === 0) return false;
     setUpload("uploading");
-    Promise.all(picked.map((f) => api.run.dropFile(f)))
+    Promise.all(picked.map((f) => api.run.dropFile(f, lane)))
       .then((saved) => {
         setUpload(null);
         const added = picked.map((f, i) => ({