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>

server/lib/pty-attach.js

@@ -104,9 +104,16 @@ function attach(ws, runId, { cols, rows }) {
   let group = groups.get(runId);
   if (!group) groups.set(runId, (group = { driver: null, viewers: new Set() }));
   const lead = group.driver;
+  // A developer's pane is on that account's own tmux server, attached AS that
+  // account (S3: never a host client on a socket the account controls).
+  // node-pty forks natively, so drop-caps' child_process patch never sees it:
+  // the client must not keep the unit's CAP_DAC_READ_SEARCH (S3).
+  const [file, args] = require("./drop-caps").ptyArgv(
+    ...(tmux.memberAttach(runId) || [attachBinary(), ["attach-session", "-t", tmux.session(runId)]])
+  );
   // A follower attaches at the driver's size: tmux sizes the window to the
   // newest client, so its own size would resize the driver's pane first.
-  const pty = spawnImpl(attachBinary(), ["attach-session", "-t", tmux.session(runId)], {
+  const pty = spawnImpl(file, args, {
     name: "xterm-256color",
     cols: lead ? lead.cols : cols || 80,
     rows: lead ? lead.rows : rows || 24,